The effects of two amino acid analogues, viz., L-methionine-DL-sulphoximine and L-methyl-DL-methionine on growth, heterocyst differentiation and nitrogen fixation in the blue-green algaNostoc linckia have been studied with special reference to heterocyst spacing pattern. L-methionine-DL-sulphoximine strongly inhibited growth but produced an unusual number of heterocysts with changed heterocyst spacing pattern in both nitrogen-free and ammonium-containing media. L-methyl-DL-methionine was less effective than L-methionine-DL-sulphoximine. An attempt was also made to counteract the toxic effects of these analogues by supplying amino acids. Glutamine and methionine reversed the inhibitory effect of L-methionine-DL-sulphoximine while only methionine reversed the inhibitory effect of L-methyl-DL-methionine. Production of changed heterocyst spacing pattern in nitrogen-free and ammonium-containing media when supplemented with L-methionine-DL-sulphioximine suggests that ammonia may not be the inhibitor of heterocyst spacing pattern.