X-174 replicative form (RF) DNA was cleaved by restriction enzymes from Haemophilus aphirophilus (Hap) and H. influenzae (H-I strain Osaka) (HinH). Five fragments were produced by Hap and eight by HinH. The positions of all of the Hap fragments and six of the HinH fragments were mapped on the genome by heteroduplex transfection and DNA-DNA hybridization methods. In addition, fragment sizes of X-174 RF DNA and S13 RF DNA by Hap and HinH were compared. Considerable differences of the size of the fragments produced from these closely related phage genomes were observed.