We evaluated the median lethal concentrations (LC50s) of the pyrithione (PT) antifoulants copper pyrithione (CuPT) and zinc pyrithione (ZnPT) to a teleost, red sea bream (Pagrus major), and a crustacean, toy shrimp (Heptacarpus futilirostris). The 96-h LC50 values of CuPT and ZnPT, on the basis of actual concentrations, were 9.3 and 98.2 μg/L, respectively, for red sea bream and 2.5 and 120 μg/L, respectively, for toy shrimp. Histological observations revealed that the secondary lamellae of the gill filaments of the experimental fish were heavily damaged after exposure to the PTs, suggesting that fatal hypoxemia was one cause of death. Because CuPT and ZnPT are usually used in combination with Cu, we also estimated the joint toxicities of the PTs with Cu using the LC50 values of the PTs and those of Cu (84.4 and 113 μg/L for red sea bream and toy shrimp, respectively). The results suggested that the joint toxicity of the ZnPT and Cu mixture is more than the additive toxicities of CuPT and Cu, especially in toy shrimp. The enhancement of toxicity in the mixture was inferred to be caused by conversion of ZnPT to the more toxic CuPT in the presence of Cu.