A study to investigate the effects of toxicity of malathion in different levels (0, 100, 250, 500, 750 and 1000 ppm) on fermentation parameters (rumen pH, rumen ammonia, gas production and in vitro dry matter disappearance) was carried out according to in vitro procedure. The study indicated that high levels of malathion (500, 750 and 1000 ppm) significantly (p<0.001) inhibited the in vitro dry matter degradability (IVDMD) for 24 and 96h after incubation, but the effect of charcoal on IVDMD was insignificant after 24h incubation exception of 96h incubation. Also NH3-N decreased after 24 and 96h incubation compared to the control group (P<0.0001). The pH of rumen fluid significantly (p<0.05) increased after 24h incubation, but there was not any significant effect for this factor after 96h incubation. With increasing the levels of malathion, cumulative gas production for 24, 48 and 96h after incubation decreased (p<0.01). With applying the charcoal, some fermentation parameters such as cumulative gas production for 24, 48 and 96h incubation increased, but there was a high significant effect for the NH3-N and pH at different times. The results showed that malathion generally had a toxic effect on fermentation parameters (exception of pH for 96h incubation) but to some extent charcoal as a toxin binder could be effective for decreasing of these toxic effects in this experiment

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