Using DNA clones, the physical distance between the linked genesnov andstr inHaemophilus influenzae was estimated. Although none of the cloned inserts contained both the markers, pJ1-8StrR 13 (insert of 18·7 kb) includedstr gene at one end and part ofnov gene at the other end of the insert. By EcoRI restriction analysis and by Southern hybridization, the distance between the two EcoRI sites, cutting at which inactivates the two genes, was estimated to be 17·7 kb. A single continuous EcoRI fragment (containing 4EcoRI sites within it) carrying both the genes intact would need to be 20·4 kb in size. These estimates were confirmed independently using different clones ofnovr andstrr alleles as probes for hybridization with BamHI-digested chromosomal DNA.