A dual flow continuous culture system was evaluated for its ability to simulate rumen fermentation in vivo. Soya bean meal, whole soya beans and whole soya beans extruded at 132 or 149° C, provided 50% of total crude protein in diets comprised of 52% grain mix, 36% maize silage and 12% alfalfa hay (dry matter basis). These four diets had previously been studied with four ruminal and intestinal cannulated lactating Holstein cows in a trial of 4 × 4 Latin square design. To conform with the results of this trial, fermenters were maintained at a liquid dilution rate of 0.096 h-1, solids dilution rate of 0.055 h-1 and pH of 6.25. There were three experimental periods, each lasting 8 days with effluent collected on the last 3 days. A Bonferroni t-test was used to compare results obtained in vitro with those obtained in vivo. Results from the two studies were similar in true organic matter digestibility and crude protein and amino acid degradability. Individual amino acid flows were higher with the diets containing heat treated (extruded) soya beans in both studies. Results indicate that the dual flow continuous culture system can provide a reasonable estimate of rumen fermentation.