ABSTRACTUnder conditions of starvation for fixed nitrogen, cells of the filamentous cyanobacteriumAnabaena variabilisKütz, degrade much of their protein prior to heterocyst differentiation. Cells starved for a source of fixed nitrogen initially degraded about 2% of their protein per hour; by 24 h after nitrogen stepdown about 40% of the protein was degraded. Most of the acid‐soluble radiolabeled material was excreted into the medium. Proteolysis was completely inhibited by chloramphenicol, by cyanide, or in the dark, hut was only partially inhibited in the presence of dichlorophenyl dimethylurea. Methionine sulfoximine (MSX) (an inhibitor of glutamine synthetase) in the presence of ammonia caused heterocysts to form. MSX treated cells degraded protein; however, the amount of protein degraded was much less than in cells starved for ammonia. Glutamine, which can serve as a nitrogen source for this strain, did not prevent starvation‐induced proteolysis and did not prevent the differentiation of heteroc