Although it is known that several phage races, capable of developing on a certain strain of Streptococcus cremoris, exist in the surroundings of commercial cheese factories in New Zealand, only one specific race is invariably isolated from cheese or cheese whey when the culture is used as a starter. Laboratory and field evidence suggests that this particular race has a special affinity for the organism, and that its presence interferes with the growth of other races which could develop in its absence.