The possibility that exposure to bifenthrin induced an outbreak of teratospermia at a bull stud was investigated. Semen evaluation and clinical examination of reproductive organs were performed weekly on hospitalized sentinel cases and at critical intervals on bulls remaining at stud. During the 13 days following exposure to bifenthrin, semen quality was poor, with significantly decreased progressive motility, high percent of distal midpiece reflexes, and reduced ejaculate volume. However, clinical examination of bulls revealed no gross abnormalities of reproductive organs. Evaluation 26 days following exposure showed overall improvement in semen quality, with progressive motility increased, percent distal midpiece reflexes reduced, and ejaculate volume trending upwards. Clinical examination continued to demonstrate no abnormalities of reproductive organs. Semen quality recovered to levels nearly equivalent to those observed before exposure and was similar when re-evaluated at 64-days postexposure. These data suggest a potential deleterious effect of bifenthrin on epididymal, but not testicular function.