Diazinon is a widely applied agricultural pesticide whose effect importantly on the environment and the possible contamination of surface waters has led to increased interest in toxicological studies. Crayfish, as an ecologically important benthic macroinvertebrate, seems to be an appropriate model organism for such assessments. Acute toxicity tests were carried out on three crayfish age groups: young-of-the-year (total length=25.0±4.9 mm), juvenile (total length=56.5±3.8 mm) and adult (total length=83.5±5.7 mm). Young-of-the-year crayfish were found to be the most sensitive to diazinon (96 h LC50 = 0.15 mg L-1), followed by juvenile crayfish (96 h LC50=0.27 mg L-1), and adults (96 h LC50=0.51 mg L-1). Crayfish were highly sensitive to diazinon. A delayed effect of Diazinon 60EC on adults was detected (144 h LC50=0.44 mg L-1) suggests functional damage from the use of sublethal concentrations.