Fluorodeoxyuridine (FUdR), an inhibitor of DNA synthesis, was examined for its ability to prevent a synchronous population of C. elegans from reproducing without otherwise interfering with the organism's post-maturational development and aging. When a synchronized population was exposed to 400 micrometer FUdR just as the population reached sexual maturity, the FUdR induced complete sterility within five hours by preventing eggs from hatching. Any larvae that hatched from eggs made before the FUdR was added remained small in the presence of FUdR and were easily removed by filtration or sedimentation. FUdR-sterilized adults showed no morphological abnormalities. Age-associated changes seen in controls also occurred in FUdR-treated worms, including atrophy of the gonads, increased pigmentation, sluggishness and increased transparency. Life span was not shortened by FUdR treatment. Our observations suggest that treatment with FUdR under carefully controlled conditions is a reasonable way to maintain synchronously aging populations of C. elegans.