Abstract 1. Acetylcholine hydrolase (AChE) in abdominal ganglia of the crayfish, Procambus clarki was studied in the animals kept in hot, warm and cold was studied in the animals kept in hot, warm and cold temperatures. 2. Arrhenius plots of log V max vs T − for crayfish AChE changed linearly between 15–35%C. The apparent activation energy was 2.28, 1.84 and 1.52 kcal/mol in the hot-, warm- and cold group of crayfish, respectively. 3. AChE affinity for AthCh (acetylthiocholine) varied with temperature, and the minimum apparent K m value was attained at the environmental temperature of the crayfish. 4. These temperature-dependent kinetic properties of ganglia AChE were discussed in relation to the role of ACh as a neurotransmitter in Procambarus.