Several lytic and lysogenic actinophages were isolated from soil samples infected with Streptomyces cattleya and S. venezuelae. The morphologies and some biological properties of the phages, and the physico-chemical characteristics of their DNAs, were compared. Electron micrographs indicated that all the phage heads were of an icosahedral form, but head size and length of the tail varied. Two of the phages had a broad host range; the other isolates could lyse only a limited number of species. The molecular sizes of the phage DNAs were between 32.2 and 98.5 kb as estimated by electron microscopy and restriction enzyme analysis. The same study also indicated that one of the DNA species contained cohesive ends. The G + C content of the DNAs ranged between 45.1 and 74.2 mol % as estimated from melting studies. Sedimentation velocity experiments implied that several of the phage DNAs were probably heavily glycosylated or methylated. These modifications might explain the partial or slow digestion of some of the DNAs by several of the 23 restriction enzymes tested. Protoplasts of the appropriate Streptomyces strains could be efficiently transfected with phage DNA in the presence of 25% (w/v) polyethylene glycol (mol. wt 6000).