This study was conducted to examine the in vitro fermentation pattern after incubation of rumen bacteria or protozoa with different levels of peptide as a substrate. Casein trypticase was used as peptide source and was added to the medium to make final concentration of 0, 0.2 and 0.4 %. 1. Ammonia concentration was significantly different between bacteria and protozoa. Before 6 hrs incubation, ammonia concentration was higher in bacteria than in protozoa, but after 12 hrs incubation, reversed effect was shown. A similar trend was observed in soluble protein concentration in the medium. 2. Peptidase activity was not significantly influenced by microbial type or peptide supplementation level. 3. Individual VFA and total VFA concentration were higher when bacteria were microbial source compared to protozoa. Acetate/propionate ratio was higher in protozoa when peptides were not supplemented. but higher in bacteria when peptides were added. 4. Gas production/total VFA was higher in protozoa at most incubation time which may imply the efficiency of energy production was lower in protozoa than in bacteria.