Ten Pseudomonas phage were isolated by sewage enrichment. Five psychrophilic and five mesophilic phage were selected for a description of some of their biological properties. In addition to growth on psychrophilic hosts, the psychrophilic phage studied were also able to grow on a mesophilic host within its growth temperature range. Latent periods for psychrophilic phage at 3.5 C were 6 to 12 hr and at 25 C were 30 to 60 min. Mesophilic phage had a latent period of 85 to 190 min at 25 C and 35 to 85 min at 37 C. Psychrophilic phage were significantly more heat-sensitive than the mesophilic phage. Of all the parameters studied, only thermal sensitivity correlated with growth at 3.5 C. Phage used in this study had a deoxyribonucleic acid base composition ranging from 39.6 to 68.2% guanine plus cytosine, deduced from melting temperature measurements.