Abstract: Present study was undertaken to determine the median lethal dose (LD50) of β-cyfluthrin (β-cyf), a synthetic (type II) pyrethroid pesticide in male and female Swiss albino mice. The calculation of LD50 is an initial screening step in the assessment and evaluation of the toxic characteristic of a chemical substance. The selected single doses of β-cyfluthrin based on the pilot study were 200, 225, 250, 275 and 300 mg/kg b.wt. for male and 350, 375, 400, 425 and 450 mg/kg b.wt. for female animals. Pesticide was dissolved in corn oil and administered orally to different groups of male and female mice, consisting of 10 animals in each group. Signs of toxicity and possible death of animals were monitored upto 96 h to calculate the median lethal dose (LD50) of β-cyf. The results obtained in this study suggest that oral LD50 of the β-cyf dissolved in corn oil was found to be 260.01±30.73 and 407.38±24.68 mg/kg b.wt. in male and female Swiss albino mice respectively.