Allelopathy of diterpenoids extracted from plants of the genus Robdosia on three common species of soil ciliates, Colpoda inflata, Colpoda cucullus and Euplotes muscicola, was studied by acute toxicity test, sub-lethal effect test and morphological observation. Acute toxicity test showed that there was remarkable toxicity of the diterpenoids on the individuals of the three soil ciliate species, and there was close correlation between toxicity and concentration of the diterpenoids. 12 h-LC₅₀ values of the diterpenoids on the individuals of C. inflata, C. cucullus and E. muscicola were 161.40 mg L^-1, 94.80 mg L^-1 and 83.70 mg L^-1 respectively, and 24 h-LC₅₀ values were 114.90 mg L^-1, 92.30 mg L^-1and 65.80 mg L^-1 separately. Sub-lethal effect test of soil ciliates suggested that there existed significant inhibition of the diterpenoids on population growth of the three ciliates with dose-dependant relationships, population density and growth rate of the test group was obviously lower than that of the control group. Morphological observation indicated that diterpenoids affected the body shapes of the three ciliates and made them shorter and thicker, and the higher the concentration of diterpenoids, the greater the affection. The results are of great significance for understanding the functions of ciliates and their relationships to other organisms, and for the application of allelopathy in biological pest control in the soil ecosystems.