Earthworms, Eisenia fetida, were treated by surface exposure to the fungicide benomyl at various stages of posterior segmental regeneration. Teratogenic effects of benomyl were observed when worms were treated 7-11 days after amputation (i.e. during the normal period of segmental replication), but not during days 1-5,13-17, or 19-23. Teratogenic effects included a reduction in the number of regenerated segments, an increased frequency of segmental groove anomalies, and a variety of monstrosities. The effects of benomyl treatment on the number of regenerated segments and frequency of anomalies were dose-dependent within a narrow concentration range (approximately 0.2-5.0 mg litre-I); at higher concentrations (e.g. 25 mg litre-') teratogenic effects were less frequent because the onset of segmental delineation was delayed until well after exposure. Non-invasive electrophysiological recordings from treated worms indicated that functional integrity of giant nerve fibre conduction pathways was established in all anomalously regenerated tail segments, except in a two-tailed monstrosity. The evidence supports the hypothesis that these teratogenic effects derive from an antimitotic mode of action of benomyl on segmental regeneration.