Sodium salt of 2,4-dichlorophenoxyacetic acid (80% active ingredient), commonly applied for the control of aquatic weeds, was used to observe its effect on the growth and nitrogen fixation of a heterocystous bloom forming blue-green algaAnabaenopsis raciborskii. A concentration of 10Μg per ml of 2,4-D showed stimulation of growth and nitrogen fixation and these were almost unaffected in presence of its 100Μg per ml in the medium. The alga could tolerate up to 800Μg per ml in liquid culture media with and without nitrate nitrogen and up to 90Μg per ml on to agar plates. Nitrogen fixation was inhibited in presence of its higher concentrations.