Two independently isolated temperature-sensitive bacteriophage that are specific for enterobacterial hosts harboring HI and HII plasmids were characterized to determine if any identifiable differences existed between them. The traits examined included adsorption pattern of phage to H pili, bacteriophage size, sensitivity to chloroform, RNA strandedness, reaction with F-specific antiphage serum, virion protein pattern, temperature range of lytic ability, and plaque morphology. No differences between the phages were observed for any of the features analyzed. Ecological questions on the origin and maintenance of temperature-sensitive phages are discussed.