Seventy-six Cheddar cheese starter cultures were collected from five cheese plants located in New York, Wisconsin, and Illinois. Several single colony isolates from each starter culture were purified and identified to species on the basis of six biochemical reactions. Thirteen of the 76 starter cultures were identified as mixture of Strepcococcus lactis and S. cremoris and 63 of the starter cultures were single species. Plasmid profiles and resistance to five phages (ML3, C2, TRM, 18-16, and C5W6) were determined for all 450 isolates. One hundred and thirty-six isolates that constituted a representative cross-section of commercial strains were selected upon their plasmid and phage-resistance profiles. These strains were used as indicators to test phage activities of 57 wheys collected from four of the five cheese plants. Wheys from different cheese plants showed varying levels of phage activity. Wheys from the plant that had the most acute phage problem completely lysed at least 25% of the 136 strains. Most of the sensitive bacterial strains of the 136 isolates were lysed by wheys from two individual plants. The percentage of strains from each plant resistant to all 57 wheys ranged from 9 to 43.