Abstract: Phorbol Esters (PEs) from Jatropha curcas seeds are known to have toxicity effects on various animal tissues but little is known about their effects on microorganisms residing in the gut systems of animals when the compounds are consumed. Hence, in the present study, the effects of PEs isolated from Jatropha meal on rumen microbial activities were determined. The rumen microbial fermentation, enzyme activity, microbial protein synthesis, rumen microbial population and the morphology of rumen microbes were examined in vitro in the presence of PEs at 0, 1.5, 3, 4.5 and 6% with 200 mg of Panicum maximum as substrate. The results showed that PEs at 1.5 and 3% did not affect the rumen microbial fermentation activities including, dry matter digestibility, in vitro gas production at 24 h, total VFA, acetic: propionic ratio and cellulolytic enzyme activity (carboxymethylcellulase, filter paperase). However, microbial activities were significantly (p<0.05) suppressed at 4.5 and 6% PEs. On the other hand, microbial protein synthesis and microbial population (general bacteria, fungi, protozoa, methanogens, archaea and major cellulolytic bacteria) values were significantly (p<0.05) reduced at all levels of PEs. Morphological examination using scanning electron microscopy revealed the aggregation and membrane disruption of the rumen microbes. The results of this study showed that, although PEs affected the rumen microbial activities in a dose-dependent manner with no significant adverse effects up to 3% but the microbial population was significantly suppressed (p<0.05) at all levels of PEs.