Regeneration of caudal fins in Fundulus was retarded by treatment with 10 μg/liter (10 ppb) of DDT, Malathion, Parathion, and Sevin. Although DDT was quite toxic to the fish, it did not retard regeneration as much as the other insecticides, showing significant retardation only after 2 weeks. The other three which retarded regeneration to a greater extent are all acetylcholinesterase inhibitors. There may be a relationship between acetylcholinesterase inhibition and fin regeneration, although the effect on regeneration was probably not directly related to this enzyme's inhibition.