The effects of an organophosphorous insecticide, methamidophos, on the pest control potential of the spider Hylyphantes graminicola (Sundevall) (Araneae: Linyphiidae) were investigated in the laboratory with the fruit flies (Drosophila melanogaster Meigen). The influence of methamidophos on predation by H. graminicola was very obvious in female spiders, which preyed on fewer prey in the 8 h after exposure to the insecticide but subsequently recovered. On the other hand, the predation rates in male spiders were not affected by the insecticide within 24 h of treatment. However, a 10% lethal dose (LD10) of methamidophos resulted in an enhanced predation rate per day for male spiders, whereas a 50% lethal dose reduced the predation rate. In addition, it was shown that the functional response of H. graminicola to the fruit fly was a type II response, and the type of functional response of insecticide-treated females changed from type II to type I, with no change in the response of male spiders. The attack rate of males treated with the LD10 dosage of insecticide was significantly higher than the controls, which suggests that the insecticide stimulates the performance of spiders. Prey utilization of males treated with low doses of insecticide was lower than the control, which indicates that the insecticide did not result in these spiders eating more prey, but killing more.