AbstractEarthworms,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−1); at higher concentrations (e.g. 25 mg litre−1) 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 seg
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