Anabaena doliolum Bhar. survived up to 3 mg/litre endosulfan on agar plates. Inhibition was visible from the beginning of growth at all tested concentrations; the LC50 was 2.15 +/- 0.07 mg/l (p = 0.05). Though the growth and pigment inhibition at concentrations < 1.0 mg/l was not significant, these were severe at all other tested concentrations (> or = 1.0 mg/l). The pesticide was detoxified by A. doliolum at concentrations of 1.5, 2.5, and 3.0 mg/l. The growth rates were more than 90% of the control at 2.5 and 3.0 mg/l, while growth acceleration was observed at 1.5 mg/l after the third inoculation. The initial cell density was also found to be an influential factor in regulating the toxicity of the pesticide, the toxicity being inversely related to the cell density.