The toxicity of triphenyltin has been determined after its acute oral and intraperitoneal administration in rats, guinea-pigs, rabbits, mice, and hens, after feeding it to rats and guinea-pigs, and after its application to the skin of guinea-pigs. The guinea-pig was the most sensitive species and its growth was inhibited by as little as 1 p.p.m. triphenyltin acetate in the diet. With higher concentrations in the diet the relationships between the dose and the survival time and between the amount consumed and the acute oral LD 50 indicated that triphenyltin accumulated in the guinea-pig. Compared with those of triethyltin, the effects of triphenyltin appeared relatively non-specific, but there was evidence of an action on the central nervous system although cerebral oedema did not occur. The effects of triphenyltin were quite different from those of diphenyltin which acted mainly as an irritant. Triphenyltin did not readily penetrate unbroken skin. The use of triphenyltin as an agricultural fungicide is considered in the discussion.