The objective of this research was to evaluate the in vitro degradability and gas production from different byproducts of the domestic biodiesel industry (Glycine max, Brassica napus L., Helianthus annuus and Raphanus sativus L.) using four increasing levels (0, 30, 50 and 70%) of replacement of Pennisetum purpureum. The inoculum for the in vitro incubations was obtained from three Holstein cows with rumen fistulas. Gas production was measured at 3, 6, 12, 24 and 48 hours of incubation. The experimental design was a completely randomized 4 × 4 factorial arrangement, with byproducts and their levels being the factors. The results showed a significant effect (P ≤ 0.05) of all byproducts. A higher level of degradation was observed following 48 hours of incubation with Glycine max compared with other byproducts, but it did not reduce gas production; Raphanus sativus was the most efficient byproduct to reduce the total gas production. The lowest degradation was obtained with sunflower. All the studied byproducts may be used in the diet of ruminants as potential protein supplements.