SUMMARY The lytic bacteriophage ~SW-I 4 was isolated from sewage using Pseudo- monas acidovorans no. I4 as host. The phage had an icosahedral head some 85 nm. in diameter and a contractile tail some I4o nm. long. ~W-I4 formed plaques on only a few strains of P. acidovorans. The phage gave biphasic ab- sorption kinetics, with an adsorption constant of I'9 x io -9 ml./min. The latent period was 6o rain. and the burst size was about 3oo. The burst size was dependent upon culture age. The kuv. for inactivation of ~W-I 4 was 4"35 min--1. P. acidovorans was shown to possess a potent photoreactivating system. The heat of inactivation of~W- 14 was calculated to be 75,7oo cal./mole. The phage gave biphasic thermal inactivation kinetics at 55 ° and 6o ° but not at 65 °. The phage mutated spontaneously to a different plaque type. This mutation affected the adsorptive properties, the thermal sensitivity and the burst size of the phage.