The nif cistrons indentified by complementation analysis in the preceding paper (Dixon et al., 1977) were mapped with respect to hisD and to each other other by P1 cotransduction and three-factor reciprocal crosses. The order obrained was hisD nifB nifA (nifL) nifF nifE nifK nifD nifH. Analysis of hisD2-nif cotransduction data by the Wu equation (Wu, 1966) suggested that the nif genes are divided into two clusters: a his-proximal cluster comprising nifBA(L)F and a his-distal group of nifEKDH.