Catgut Biothread – Ultra-Realistic Fly Tying Material Catgut Biothread is a premium fly tying material developed for creating exceptionally realistic nymph and emerger bodies. Its unique behavior in water and unmatched visual realism have made it a trusted choice among fly tyers looking to create lifelike insect imitations. When dry, Catgut Biothread appears opaque and solid in color. Once submerged, it transforms into a milky translucent, glossy, soft, and flexible material. As it absorbs water, it also increases slightly in volume, producing a naturally plump body profile that closely mimics living aquatic insects. What truly sets Catgut Biothread apart is its appearance underwater. Flies tied with this material develop lifelike texture, depth, and natural translucency that are difficult to achieve with traditional body materials. The material has also been specially treated to improve underwater durability, providing reliable performance over repeated use. How It Works Before tying, simply soak the desired length of Catgut Biothread in warm water for 3–5 minutes. During soaking, the material becomes transparent, flexible, and easy to wrap. Once the finished fly dries, the body returns to its original opaque appearance. By varying the color of the underbody thread, you can create a wide range of subtle natural effects as the color shows through the translucent material, giving you even more flexibility when matching different insects. Product Details Approximately 50 cm (20 in) per pack Available in multiple diameters and a wide range of colors Size Guide Extra Small (0.18 mm): hooks #16–20 Small (0.30 mm): hooks #14–16 Medium (0.50 mm): hooks #12–14 Large (0.70 mm): hooks #8–10 Best Uses Ideal for tying: Realistic nymph bodies Emerger patterns Traditional salmon fly loops (especially XS and Small diameters) How to use Catgut Biothread Tying a Classic Micro Nymph with Catgut Biothread Fly Tying a Catgut Caddis Pupa For a full list of ingredients for this recipe, go here.
