Toxicities o1 12 chemical insecticides to wild‐caught tsetse flies,Glossina palpalis palpalisin Nigeria were measured by topical application. Median lethal doses (as ng per fly) at 48 h after application for older flies, males (females), were: DDT 331 (700), dieldrin 14 (46), endosulfan 8 (15), malathion 193 (273), fenitrothion 47, pirimiphos methyl 31 (45), tetrachlorvinphos 14 (33), sumicidin 28 (58), neopynamin 12 (15), sumithrin 10 (8), permethrin 4.4 (5.9), deltamethrin 0.45 (0.94). Lethality expressed as dose per whole fly can be termed effective toxicity/tolerance and those expressed as dose per weight unit of fly can be termed intrinsic (or true) toxicity/tolerance. Generally, effective tolerance was greater for females than males, especially for older flies. However, intrinsic tolerance was often about equal for the sexes or greater for males. Regardless of sex, effective tolerance increased with increasing fly age and intrinsic tolerance increased during later ageing but not during early ageing. This suggested that protecting mechanisms developed during ageing. Flies fed a few hours before treatment in 1979–1982 were more tolerant of three organochlorines and three organophosphates than flies fed the day before treatment in 1974–1975, probably due to diversion of poisons from sites of action into inert undigested blood. However, results and statistical analysis suggested a slight increase in true tolerance of males to DDT during the intervening years. Continued monitoring of tolerance in the fly populations is recommended.