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In the filamentous cyanobacterium, <i>Anabaena</i> sp. PCC 7120, single heterocysts differentiate at semi-regular intervals in response to nitrogen stepdown. HetR is a principal regulator of heterocyst differentiation, and <i>hetP</i> and <i>hetZ</i> are two genes that are regulated directly by HetR. In a <i>hetR</i> mutant generated from the IHB (Institute of Hydrobiology) substrain of PCC 7120, heterocyst formation can be restored by moderate expression of <i>hetZ</i> and <i>hetP.</i> The resulting heterocysts are located at terminal positions. We used a tandem promoter, P<i>_rbcL</i>P<i>_petE</i>_, to express <i>hetZ</i> and <i>hetP</i> strongly in the <i>hetR</i> mutant. Co-expression of <i>hetZ</i> and <i>hetP</i> enabled the <i>hetR</i> mutant to form multiple contiguous heterocysts at both terminal and intercalary positions. Expression of <i>hetZ</i>, alone resulted in terminally located heterocysts, whereas expression of <i>hetP</i>, alone produced enlarged cells in strings. In the absence of HetR, formation of heterocysts was insensitive to the peptide inhibitor, RGSGR.