Technical Abstract: Preliminary data suggests that level of crude protein (CP) supplementation may direct species selection in mixed species pastures by grazing ruminants through post-ingestive feedback of ruminal by-products. A dual-flow continuous culture fermenter system was used to investigate the effect of increased CP supplementation on ruminal fermentation of a pasture-based diet. Treatments were: 10%, 12%, 14%, or 16% CP supplemented (30 g dry matter (DM)**d) to an orchardgrass-based diet (45 g DM**d). Diets were fed to fermenters at 0700, 1030, 1430, and 2030 h. Apparent ruminal digestibilities of DM, organic matter (OM), and neutral detergent fiber were not affected by supplemental CP and averaged 49.4%, 56.2%, and 80.4% across treatments, respectively. True DM and OM digestibilities were also not affected by supplemental CP, averaging 71.0% and 71.9% across treatments, respectively. Mean rumen pH was lower for the 10% supplemental CP than for the higher supplemental CP diets (5.99 vs. 6.38 mean for other treatments). Total volatile fatty acid concentration was not affected, averaging 103.0 mol**100 mol across all treatments. Total N intake increased with increasing supplemental CP diet, ranging from 1.76 to 2.00 g**d. The N flows for non-NH3-N, bacterial N, and dietary N were not affected, averaging 1.38, 1.30, and 0.46 g**d, respectively. Efficiency of bacterial protein synthesis per kg of DM or OM truly digested was not affected by supplemental CP level, averaging 22.9 and 21.7 g N, respectively. Supplementation of high-quality pasture diets with different levels of crude protein did not affect herbage digestion and nutrient supply to the lower digestive tract. Principals learned through these in vitro methods must be tested in vivo to account for post-ruminal consequences of diet selection and supplementation strategies.