The aim of this study was to reveal the effect of diazinon on the rat spermatozoa motility characteristics using the computer-assisted semen analysis (CASA). Motility, progressive motility, DAP, DCL, DSL, VAP, VCL, VSL, STR, LIN, WOB, ALH, and BCF after the diazinon i.p. administration of 20 mg/kg b.w. were evaluated. 36 hours after the diazinon administration, only slight decrease in VCL, DCL and increase in percentage of progressive motility in the diazinon-treated group. Significant decrease (P<0.01) was only observed in BCF in diazinon-treated group. Computer-assisted semen analysis (CASA) of rat sperm motility showed that acute diazinon administration slightly affected the rat sperm motility which can be the first step in the decreased fertilization capacity caused by pesticides. Further investigation of reproductive effects of diazinon is needed.