The metabolic fate of phosphoric acid triesters in the goldfish (Carassius auratus) were clarified through some experiments in vitro and in vivo. Tributylphosphate (TBP) was converted to dibutyl 3-hydroxybutyl phosphate (TBP-OH) and dibutyl phosphate (DBP) by incubation with goldfish liver microsomes. This reaction was Nicotinamide Adenine Dinucleotide Phosphate (Reduced form) (NADPH)-dependent and presumed to be an oxidation by mixed function oxidase (MFO). The microsomes did not metabolize tris (2-chloroethyl) phosphate, tris (1, 3-dichloro2propyl) phosphate (TDCPP), and triphenylphosphate (TPP) as well as TBP. The soluble fraction of goldfish liver contained an enzyme that acted specifically on TDCPP. The enzyme was glutathione (GSH)-dependent and had an optimum pH at 8-9, and it was concluded to be glutathione S-transferase (GSTase), which was stable at 45°C. GSTase in the goldfish had higher activity than the rat enzyme only toward TDCPP in several substrates tested. The superiority of goldfish GSTase to the rat enzyme might come from the difference of their rate constants, K_m values on TDCPP. The GSH contents in the liver and whole body of the goldfish were significantly decreased by exposure to diethyl maleate (DEM). Pretreatment with DEM increased the assumulation of TDCPP in the fish 6.6-fold but scarcely affected TBP and TPP contents. This results indicate that GSTase is concerned with in vivo metabolism of TDCPP. Killifish (Oryzias latipes) absorbed TBP from water and metabolized it to TBP-OH, butyl bis-(3-hydroxybutyl) phosphate and DBP, which are the same as the metabolites obtained with MFO in the goldfish liver.