WE have reported the isolation and partial characterization of a novel lipophilic glycoside which is apparently specific to nitrogen-fixing blue-green algae1. During these studies we noticed that a culture of Mastigocladus laminosus which had been grown in a medium containing ammonium salts synthesized no detectable quantities of the glycoside. More recently we have observed that the glycoside is nevertheless present when this organism is cultured on a medium containing no combined nitrogen, and similar observations have been made with Anabaena cylindrica cultured in the two sets of conditions (Fig. 1). The formation of heterocysts by these and other nitrogen-fixing blue-green algae is inhibited by the presence of ammonium nitrogen in the culture medium2 and so it seemed possible that the glycoside might be specifically located in the heterocyst. To test this hypothesis, an investigation was made into the lipid content of heterocysts prepared from cultures of A. cylindrica.