Extracts from ten species of Cyanophyta were applied separately to axenic cultures of Trichomonas hominis, T. muris, and T. augusta. During an incubation period of seven days, extracts from the ten algae inhibited the growth of two or more of the protozoan parasites at the interval corresponding to the maximum growth phase of the controls. Lyngbya sp. extract at a concentration of 0.45 ml/ml medium was most effective, with 98% inhibition in T. hominis and 100% in T. muris. Extracts from the two species of Anabaena were least effective.