When growing on N(2) as sole nitrogen source, the filamentous cyanobacterium Anabaena sp. PCC 7120 forms N(2) fixing heterocysts in a semi-regular pattern. To identify genes involved in heterocyst differentiation we characterised five transposon-generated mutants that were not able to form mature heterocysts. After recovering the transposon together with the flanking region of the Anabaena chromosome the affected genes were identified. Four of the genes could be involved in formation of the heterocyst-specific envelope: alr2887, encoding a probable outer membrane efflux protein, alr3698, a glycosyl transferase, all4388, a putative periplasmic polysaccharide export protein and alr5357, the formerly described gene hglB/hetM, encoding a fatty-acid synthetase. Another gene, all0049/mutS2, may be important in one of the genome rearrangements that occur during heterocyst differentiation. By transcriptional fusion to reporter genes luxAB differential expression of alr2887, alr3698 and alr5357 could be monitored during heterocyst differentiation.