2,4-Dichiorophenoxyacetic acid (2,4-D) stimulated growth and heterocyst differentiation of Anabaena sp. in nitrogen free medium at low concentration 100 µg mL^-1 while its higher concentrations inhibited both processes and 500 µg mL^-1 proved to be lethal. Dry mass and specific growth rate of algae declined with increasing concentration of 2,4-D in the range of 100-1500  µg mL^-1. Glucose slightly increased the heterocyst frequency without any lag in their differentiation. Tryptophan promoted growth of the alga and formation of heterocysts (nearly three fold). Tryptophan (50 µg mL^-1) complex medium with 1 mg 2, 4-D per ml did not produce mature heterocysts. The filaments were fragmented at the point of heterocyst development and detached heterocyst germinated in situ. Glucose and tryptophan protected the alga, its growth and heterocyst differentiation even at the lethal concentration of the herbicide.