SummaryBaseline acetylcholinesterase (AChE) activity was established in honeybees from three sources: foraging bees (Group I), upper brood nest area (Group 2), honey storage area (Group 3). AChE concentration was 70, 142·5 and 143·5 units/brain for Groups 1, 2 and 3, respectively, compared with a standard preparation of crystalline bovine erythrocyte AChE. Malathion oral LD50in 1-M sugar syrup was 0·41, 0·82 and 0·87μg/bee for Groups 1, 2 and 3, respectively, after 24 h exposure to the toxicant. AChEin vitrostudies, in which bee head homogenate was the enzyme source, gave malaoxon molar I506·4×10−11M for Group 1 and 1·9×10−10M for Groups 2 and 3. These data indicate differences in AChE concentration, rather than differences in enzymatic activity, in bee brain at different stages of maturity. Bees with less brain enzyme (foragers, Group 1) were more susceptible to organophosphorus AChE inhibitors than younger bees (Groups 2 and 3), which had higher concentrations of brain enzyme.