Technical Abstract: Flaxseed has been shown to decrease methane output when fed to ruminants in confinement. Organic dairy producers are interested in flaxseed as an alternative to other organic grains due to price, ability to grow on the farm, and availability. However, little is known regarding the effects of flaxseed on a pasture-based diet. Therefore, a continuous culture fermentor system was used to assess the effect of increasing organic flaxseed supplementation on nutrient digestibility and methane output. Treatments were: 0, 5, 10 and 15% ground flaxseed supplementation of an orchardgrass diet. Dry matter, organic matter, and neutral detergent fiber digestibilities decreased linearly with increasing supplemental flaxseed. Mean ruminal pH and concentration of total volatile fatty acids were not significantly affected by increasing the dietary concentration of flaxseed. However, molar proportions of the individual volatile fatty acids acetate and propionate increased while those of butyrate and valerate decreased with increasing flaxseed supplementation. Methane output decreased as supplemental flaxseed increased from 0 to 15% of diet. Dry matter, ammonia concentration, crude protein digestibility, and bacterial nitrogen synthesis did not differ across treatments. Incremental ground flaxseed supplementation of a pasture-based diet resulted in a corresponding decrease in methane output. However, nutrient digestibility also decreased with flaxseed supplementation, which, at the cow level, could result in decreased dry matter intake and/or milk production. Additional research is necessary at the cow level to determine the optimal level of flaxseed supplementation to balance nutrient digestibility with methane output.