Despite strict standardisation of cultural and bioassay procedures, significant results were attained with difficulty because of great variation in mortalities of batches of adult Ptinus tectus Boield. exposed to insecticide deposits. In tests using lindane, the variation between weekly batches was reduced by selecting beetles within the weight range 1·8–2·4 mg, but variation between replicates in a batch was unaffected. This latter variation was found to stem both from the weights and from the sexes of the beetles; in general, heavier beetles were less susceptible than lighter ones and females were less susceptible than males at comparable weights.