The organophosphate insecticides, parathion, monocrotophos, dicrotophos, and Gardona® [2-chloro-1-(2,4,5-trichlorophenyl) vinyl dimethylphosphate], were highly toxic to adult alkali bees,Nomia melanderiCockerell. The topical LD50values for these ranged from 0.83 (monocrotophos) to 2.69 (Gardona) micrograms per gram of bees. Two carbamate insecticides, however, were much less toxic. The LD50values for these ranged from 47.90 [Landrin® (4:1 mixture of 3,4,5– and 2,3,5–isomers of trimethylphenyl methylcarbamate)] to 86.37 (carbaryl) micrograms per gram of bees.The alkali bees were fed and topically treated with the drugs SKF 525A (2-diethylaminoethyl 2,2-diphenyl pentanoate) and chlorcyclizine [N-methyl-N-(4-chlorbenzhydryl) piperazine] to determine if they could protect the bees from parathion. Chlorcyclizine made the bees generally more susceptible, whereas SKF 525A either had no effect or protected one sex without protecting the other. The protective effects with SKF 525A were small as well as inconsistent and therefore such drug treatments probably have little potential for protecting alkali bees from parathion in the field.Electron micrographs of the gut tissue of the insects showed that chlorcyclizine and SKF 525A had significant effects upon the cellular ultrastructure. Some of these effects were similar to those observed in other insects treated with different drugs in which detoxication measurements or biological assay tests had shown that the drug had limited potential for induction of protective detoxication mechanisms.