Molinate is a thiocarbamate herbicide used for weed control in rice fields. Since the late 1970s, findings from reproductive toxicology studies of rats have led to concern that molinate might affect human male fertility. Semen samples were collected from 272 formulation and production workers at three US plants. The samples were collected at the end of four alternate monitoring periods of either high or low exposure to molinate. In addition, 222 married workers provided reproductive-history information. Workers' mean exposures to molinate during the monitoring periods ranged from 12.7 µg/m3 to 210.9 µg/m3. There was no evidence that sperm and serum hormone levels were related to exposure to molinate before the study or exposure during the four monitoring periods. There was also no evidence of a molinate exposure-related effect on the ratio of observed to expected births.