OBJECTIVE To assess the mutagenic effects of imidacloprid in germ cells of Swiss albino male mice by sperm head abnormality (SHA) assay and dominant lethal test (DLT). METHODS Swiss albino mice were exposed to imidacloprid (22, 11 and 5.5 mg/kg/day) along with 3% gum acacia as vehicle control through oral route for 7, 14 and 28 days for SHA assay and for 28 days for DLT. The epididymal sperm smear in 1% eosin stain was analyzed for SHAs. In DLT, male mice were allowed to mate with females after 1, 3 and 6 weeks of end of pesticide treatment. The uterine contents of the sacrificed females were observed for live and dead implants. The analysis of test and control groups data was done by one way ANOVA at p < 0.05. RESULTS Exposure of all dose levels of imidacloprid (22, 11 and 5.5 mg/kg/day) for seven days did not induce significant SHAs while they induced significant SHAs compared with the control group following exposure for 14 and 28 days. The analysis of uterine content revealed a significant increase in the number of dead implants/female compared with the vehicle control in only those females which were mated with male mice after six weeks of treatment of highest dose level of imidacloprid. The dominant lethal mutations were observed only at spermatogonial stage. CONCLUSIONS Long-term exposure of pesticide generated SHAs even at lowest dose level (5.5 mg/kg/day for 14 days) and mutagenic effects at spermatogonial stage at highest dose level (22 mg/kg/day for 28 days).