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 make it a trusted choice among experienced fly tyers who demand lifelike results.
When dry, Catgut Biothread appears opaque and solid in color. Once submerged, it transforms—becoming milky translucent, glossy, soft, and flexible. As the material absorbs water, it also increases 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 display lifelike texture, depth, and natural shine, creating an authentic look that is difficult to achieve with traditional body materials. The material has been specially treated to improve durability underwater, ensuring consistent performance over time.
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 fly dries, the body returns to a solid-colored appearance. By varying the underbody thread color, fly tyers can create a wide range of natural effects as the color subtly shows through the wet material—offering nearly limitless design possibilities.
Product Details
- Length: approx. 50 cm (20 inches)
- Available in multiple diameters and a wide range of colors
Size Guide & Hook Recommendations
- Extra Small (0.18 mm): hooks #16–#20
- Small (0.3 mm): hooks #14–#16
- Medium (0.5 mm): hooks #12–#14
- Large (0.7 mm): hooks #8–#10
Best Uses
Catgut Biothread is ideal for:
- Nymph bodies
- Emerger patterns
- Traditional salmon fly loops (especially XS and S diameters)
For fly tyers focused on realism, translucency, and natural movement, Catgut Biothread offers a level of authenticity that elevates both the look and effectiveness of finished flies.
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.