Larval ticks were immersed in aqueous preparations of chlorphenamidine labelled with 14C in the 2-methyl position. The chemical penetrated rapidly and produced no mortality of larvae, but a quick knockdown from which at low concentrations they appeared to recover quickly. Metabolism of chlorphenamidine was relatively slow compared with that of organophosphorus acaricides in the same strain of larvae. Major metabolites, identified by paper chromatographic and countercurrent distribution behaviour, and by ultraviolet spectra, were N-formyl- 4-chloro-o-toluidine, 4-chloro-o-toluidine, and a conjugated phenolic derivative. This sequence appeared to be the metabolic route. The evidence suggested that chlorphenamidine itself was the active toxicant.