Abstract: The toxicities of cycloprothrin dissolved in wa-ter with toluene and acetone as cosolvent separately on paramecium were tested .The results show the LC₅₀ of cyclo-prothrin to paramecium caudutum is about 16.3mg/ l,while it is dissolved in water with toluene as cosolvent,or 24.0mg /l Whilt it is dissolved in water with acetone as cosolvent,It is suggested that acetone is better than toluene when cyclopro-thrin is applied.