Phage A1 isolated from the rhizosphere of Lotus corniculatus was studied. It had a very narrow host range, as it was active only against Mesorhizobium loti HAMBI 1129. Phage A1 was classified as belonging to C Bradley's group bacteriophages. The latent period of A1 was 120-130 min and a burst size 13-17 particles per cell. The nature of the phage receptor was examined. Lipopolysaccharide from the phage-sensitive strain inactivated phage A1 in contrast to LPS from the phage-resistant bacteria. Purified LPS obtained from M. loti HAMBI 1129 had a high receptor activity with PhI50 value of 0.025 mu g/ml.