If you're staring at a deep, fast-moving run on a cold morning, reaching for a 20 incher fly is usually one of the smartest moves you can make. It's one of those patterns that just looks like a meal. It doesn't try to be too delicate or fancy; it's a big, buggy nymph designed for one specific purpose: getting the attention of the largest fish in the river.
Most fly anglers have a few patterns they absolutely swear by, and for anyone who fishes stonefly-rich waters, this fly is usually at the top of the list. It's heavy, it's flashy, and it has a profile that screams "high calorie" to a hungry brown or rainbow trout. If you've ever struggled to get your flies down into the strike zone during high water, this is the pattern that solves that problem.
Why This Pattern Actually Works
The magic of the 20 incher fly isn't really a secret, but it is a bit of a masterpiece in fly design. Originally credited to Charlie Craven, it was built to mimic a stonefly nymph, specifically the larger varieties like the Golden Stone or even a small Salmonfly. But honestly, it's buggy enough to pass for a dozen different things.
Trout aren't always looking for a perfect anatomical match. Most of the time, they're looking for a silhouette and a behavior. Because this fly is typically tied with a good amount of weight—often a bead head and some lead wire under the body—it tumbles along the bottom exactly like a natural stonefly that lost its grip on a rock.
The combination of peacock herl and pheasant tail gives it a natural, iridescent shimmer. Peacock herl is basically cheating in the fly-tying world; fish just can't seem to resist that green-gold glow. When you add the flashback strip on the wing pad, you get just enough glint to catch a fish's eye in murky water without looking like a disco ball.
When to Tie One On
You shouldn't just fish a 20 incher fly year-round without thinking, but there are definitely "prime times" for it. Spring and early summer are the obvious choices. This is when stonefly nymphs are most active, moving toward the banks to hatch. When the water is a bit high or slightly off-color from snowmelt, this fly becomes a superstar.
I've found that it works exceptionally well as the "anchor" fly in a two-fly nymph rig. Because it's heavy, it acts like a sinker for your smaller dropper fly, like a size 18 Zebra Midge or a Pheasant Tail. It gets the whole rig down fast, which is usually where the big fish are lounging anyway.
Don't be afraid to use it in the fall, either. Even when there isn't a massive stonefly hatch imminent, big trout are opportunists. They remember what a stonefly looks like, and they know it's a big enough meal to be worth the energy of moving a few inches to grab it.
The Anatomy of the 20 Incher
If you're a tyer, you know that some flies are a headache to put together. Thankfully, the 20 incher fly is relatively straightforward, though it does require a bit of layering. It's all about the textures.
The Foundation
It usually starts on a long-shank nymph hook. You want something sturdy because, as the name implies, you're hoping to hook into something around the 20-inch mark. A 2x or 3x long hook gives you plenty of room to build that tapered, segmented body that stoneflies are known for.
The Body and Ribbing
The back half is usually pheasant tail fibers, ribbed with fine wire. This gives it that classic "buggy" look. But the real meat of the fly is the thorax, where you wrap thick strands of peacock herl. It creates a bulky, fuzzy profile that traps air bubbles and looks alive underwater.
The Flash and Legs
The "flashback" is typically a piece of pearl tinsel or even a bit of iridescent Thin Skin over the top of the thorax. It mimics the look of a gas bubble forming under the wing pad just before a nymph emerges. For the legs, a few wraps of partridge or even just some stiff pheasant tail fibers poking out the sides add that final bit of "twitch" that triggers a strike.
Techniques for Success
Fishing a 20 incher fly isn't just about throwing it out there and hoping for the best. Since it's a heavy nymph, your presentation needs to be dialed in.
The Dead Drift
This is the bread and butter of nymphing. You want that fly moving at the same speed as the bubbles on the surface. If your line is pulling the fly faster than the current, the trout will know something is up. Using a strike indicator is the most common way to do this, but you have to be ready. Because this fly is big, a fish will often mouth it and spit it out quickly if they feel the weight of the hook.
High-Sticking and Euro Nymphing
In faster, shallower water, I prefer high-sticking. You keep as much fly line off the water as possible and lead the fly through the pocket water. The 20 incher fly is perfect for this because its weight keeps it in the "zone" even in turbulent current. You'll feel that distinct thump when a fish takes it, which is way more satisfying than just watching a plastic bobber dip.
The "Leisenring Lift"
Sometimes, giving the fly a little bit of movement at the end of the drift works wonders. As your line begins to straighten out downstream, let the current pull the fly upward toward the surface. This mimics a nymph swimming up to hatch. I've seen trout follow a 20 incher fly for five feet before smashing it right as it starts to rise.
Why the Name Matters
Let's be real for a second—the name "20 Incher" is a bit of a marketing win. Every fly fisher wants to catch a 20-inch trout. It's the gold standard for many of us. Does the fly guarantee a monster? Of course not. But it does put you in the right headspace.
When you tie on a 20 incher fly, you're signaling that you aren't messing around with the tiny 6-inch brookies today. You're targeting the fish that live in the dark shadows under the undercut banks or the deep slots behind massive boulders. It's a confidence fly. And in fishing, confidence is about 70% of the game.
Common Mistakes to Avoid
Even with a great pattern, things can go sideways. One of the biggest mistakes people make with the 20 incher fly is not using a heavy enough leader. If you're throwing a size 8 or 10 weighted nymph on 6X tippet, you're going to have a bad time. You need a bit of turnover power to cast this thing properly, and you need the strength to pull a big fish out of the rocks once it bites. I usually don't go lighter than 4X when I'm fishing these.
Another mistake is neglecting the weight. If you aren't hitting the bottom occasionally, you aren't deep enough. If you're fishing a run and not catching anything, don't just change the fly—try adding a bit of split shot or switching to a version of the 20 incher fly with a larger tungsten bead.
Final Thoughts on the Pattern
There are thousands of fly patterns out there, and it's easy to get overwhelmed by the latest "ultra-realistic" silicone creations. But there's a reason the 20 incher fly has remained a staple in fly boxes for decades. It works. It looks like food, it gets deep, and it has just enough flash to be noticed without being obnoxious.
Next time you're at the fly shop or sitting at your tying bench, make sure you have a row of these ready to go. Whether you're fishing the Madison in Montana or a tiny tailwater in the Appalachians, a 20 incher fly is a tool you want in your kit. It's the blue-collar worker of the nymph world—it isn't always pretty, but it gets the job done and it usually brings home the big ones.