The effect of the carbamate pesticide Furadan (carbofuran, 3%) on growth and heterocyst frequency of, and N2-fixation by, nine species of heterocystous cyanobacteria belonging to the genera Anabaena, Nostoc, and Cylindrospermum was examined in test tubes. Two species (Anabaena fertilissima and Nostoc commune) were most sensitive to Furadan treatment; all levels of the pesticide decreased the growth of these two organisms. Lower concentrations of the pesticide stimulated growth of the other species. Growth of Anabaena variabilis (An 2) was increased by 100% in the presence of 2000 ?g mL-1 of the pesticide in the medium. The N2-fixing capacity of all the species, except Anabaena sp. (An 7) and Cylindrospermum sp. (Cy 2), was not decreased by up to 50% in the presence of LD50 of the pesticide determined for each species.