Know Your Core


A fly line consists of two components: a core and a coating. While the coating gets the most focus and attention from anglers, the core provides three very important contributions. The core provides the strength of a fly line, controls how much stretch there is in the line, and influences how stiff the line is. Though only a factor on a few lines, the core color can also contribute to the line's final color. For example, a clear fly line must have a clear core.



At RIO we use 5 different core materials in a multitude of strengths, types, and sizes. Each fly line’s intended use is carefully considered when we select the core material, as the end performance is based as much on the core choice as the line taper. In addition to standard, off-the-shelf cores, our R&D team have developed three custom low-stretch cores. These cores are unique to RIO and are the backbone of our finest, highest performing fly lines.


Low Stretch Cores



ConnectCore Plus is our next-generation, low-stretch core. Fly lines built on this core are ultra-smooth, have exceptionally durability, and the added benefits of incredible levels of sensitivity when casting and fishing.

ConnectCore Plus gives anglers all the benefits of a low-stretch core - enhanced sensitivity, less wasted energy and faster hook set - with a significantly longer lifespan. It also allows hand stretching to easily remove memory coils for a more direct connection to fish and fly. The highest-performing, longest-lasting freshwater lines in the world are built around ConnectCore Plus


DirectCore logo


DirectCore is our highest performing, tropical conditions core, and perfect for hot, humid climes. Lines built with DirectCore have the stiffness to resist becoming limp spaghetti out on the flats, or in the jungle, yet have no memory and are easy to straighten - ensuring your fly lines lie completely straight on the water.

DirectCore is also 25% stronger than other monofilament cores of equal size and stretches 30% less at breaking point - giving the best casting performance of any tropical line, as well as unprecedented hook sets and a faster, more direct way to land fish.




connectcore logo


ConnectCore is an ultra-low stretch core material that stretches only around 6% (compared to about 30% in regular lines). This incredibly low stretch gives increased sensitivity and performance. All RIO’S InTouch fly lines are built with ConnectCore.


Standard Cores

Tropical Core

Our standard tropical core material is stiff enough to resist wilting when temperatures are high. Lines built on these cores perform best between 75 and 110 deg F, so are a perfect choice for anglers hitting the flats or making an excursion into the jungle.

Lines built on this core material will perform in cooler conditions but will need a stretch to pull out any memory first. Like the standard coldwater core material these cores are not low stretch and will break with around 30% elongation. This core material is used in our tropical Premier fly lines.

Coldwater Core

Our standard coldwater fly lines are built on a core material that remains limp and supple in cool and temperate climates. The core can be a braided multifilament, or a single strand of supple nylon – used particularly in clear lines.

If you are fishing in temperatures between 32 and about 80 deg F, these cores will perform perfectly, though they are not low stretch - breaking with around 30% elongation. Our standard coldwater core can be found in our freshwater Premier, Avid and Mainstream fly line families.