Abstract: The glucose kinase gene glk from Brucella abortus and lacZYA genes from E. coli were positioned downstream of a cyanobacterial glnA promoter, and introduced into three cyanobacterial species in a broad-host-range plasmid. Expression of the exogenous genes was confirmed by assay of β-galactosidase and glucokinase activity. The genetically-engineered cyanobacterial strains acquired capability of heterotrophic growth on lactose. Synechocystis sp. PCC 6803(pRL2224)can grow on lactose in the light with DCMU, while Anabaena strains harboring pRL2224 are capable of heterotrophic growth in complete darkness and produce heterocysts upon induction in darkness or in the light with DCMU.