The morphology, host range, structural proteins and serological properties of Streptococcus thermophilus phages isolated from Finnish cheese plants were investigated. The results show that all the nine phages belong morphologically to Ackermann's group B1. The host-phage reactions and plating efficiency justify the division of these phages into four specificity groups. Most of the phages showed an absolute host specificity as to their plating efficiency but were not strictly specific in the adsorption to different hosts. The electrophoretic profiles of the structural proteins appeared nearly identical. Ten to eleven well separated proteins could be detected. The antiserum raised against one of the phages contained antibodies with different neutralization capacity depending on the phage. Using an immunoblotting technique, four structural proteins were detected that could bind phage antibodies.