In this experiment, fermentation parameters of a TMR ration determined with different levels of phosalone as an organophosphate pesticide (0, 100, 250, 500, 750 and 1000 ppm) and bentonite (0, 1 and 2 %DM of TMR) using in vitro batch culture and gas production techniques. The results from gas production showed that cumulative gas production for 96h after incubation significantly decreased with increasing the levels of phosalone and lowest gas production for this time was observed at 1000 ppm of phosalone. Increasing phosalone in the culture was accompanied with deceasing DM degradability and NH3-N and differences between treatments were statistically significant, so that phosalone at 1000 ppm and control groups had lowest and highest NH3-N respectively. With increasing of phosalone at different levels to the medium, pH remained constant during the 24h incubation, but with increasing the levels of bentonite (specially the 2 percent level), the pH relatively increased. The results of this experiment showed that some of fermentation parameters (such as cumulative gas production at different incubation times, NH3-N and DM degradability) decreased by increasing phosalone in culture media (0 to 1000 ppm). Supplementation of bentonite rather than the control group with no pesticide resulted to decreasing of the gas production at 96 h incubation. In coordination with these results, generally supplementation of bentonite (especially at 2% of DM) to different level of phosalone also resulted to decreasing of the cumulative gas production in different times of incubation. There was no significant effect for DM degradability by adding the bentonite to culture media between treatments, although only by adding the bentonite at 1000 ppm phosalone, DM degradability increased.