AbstractN‐acylation of aryl N‐methylcarbamates is known to reduce their mammalian toxicity considerably without adversely affecting their insecticidal activity. It has now been found that N‐acylation of several insecticidal oxime carbamates results in loss of toxicity both to insects and mammals. Kinetic data for base‐catalysed solvolysis and inhibition of acetylcholinesterase, together with published work on metabolism, are combined to provide a rationale for this unexpected observation and to account for the selective toxicity of arylN‐acetyl‐N‐met