Predominant bacteria in raw milks held at farm bulk cooling tank temperatures were gram negative rods of the following genera: Achromobacter, Aerobacter, Alcaligenes, Flavobacterim and Pseudomonas. When inoculated into raw milk and held at 38° F., most of the Pseudomonas cultures grew rapidly and steadily for the first two days and more slowly during the last two days. The Achromobacter and Alcaligenes cultures grew rapidly for the first three days and more slowly during the fourth, while the Aerobacter culture growth. Flavobacterium cultures failed to grow appreciably.