The influence of temperature on the acute toxicity of a technical formulation (86%) and pure formulation (99%) of pentachlorophenol (PCP) to less than 24‐h‐oldSimocephalus vetulusneonates was determined with 48‐h static toxicity tests. The technical grade PCP was significantly more toxic toS. vetulusthan the pure PCP (P <0.05). Sensitivity ofS. vetulusto technical PCP also significantly increased with temperature (P <0.05), but a significant temperature effect was not found with the pure PCP. The mean 48‐h LC50values for neonates exposed to technical PCP were 140 and 199 μg l−1at 22°C and 16°C, respectively, and for those exposed to pure PCP were 262 and 304 μg l−1, respectively.