In strain NE1 of Tn5-1058-mutagenized Nostoc ellipsosporum, the transposon was found within a gene whose translation product is similar in amino acid sequence to the arginine-biosynthetic protein N-acetylglutamate semialdehyde dehydrogenase encoded by argC of Bacillus subtilis. The argC reported from Anabaena sp. strain PCC 7120 hybridized to a sequence different from the one interrupted by the transposon in NE1. The newly identified gene from N. ellipsosporum was denoted argL. The argL mutation renders certain processes in strain NE1 conditionally dependent on provision of L-arginine. Heterocysts and apparent akinetes that formed in the absence of added L-arginine failed to fix dinitrogen or to germinate, respectively, and lacked granules of cyanophycin, composed of copolymers of arginine and aspartic acid. However, apparent akinetes that differentiated upon growth of the mutant in the presence of L-arginine plus nitrate formed cyanophycin granules and could regenerate a new culture.