Amitraz, 1 ,5-di(2,4-dimethylphenyl)-3-methyl-1 ,3,5-triazapenta-1 ,4-diene, labelled with 14C in the 2-methyl groups was applied to B. micro plus larvae by an immersion technique. The chemical penetrated readily but never appeared in large amounts internally due to rapid cleavage to N-2,4- dimethylphenyl-N'-methylformamidine. The expected complementary cleavage product 2,4- dimethylformanilide was not produced in equivalent quantity. However, large amounts of polar metabolite(s) were produced. Small quantities of 2,4-dimethylaniline and an unidentified non-polar metabolite were also produced. Of the identified chemicals only amitraz and N-2,4-dimethylphenylN'- methylformamidine were toxic to larvae. Piperonyl butoxide applied simultaneously with amitraz had only a slight effect on metabolism but had a three-fold synergistic effect. SKF 525-A similarly applied had a negligible effect on both metabolism and toxicity.