Ground beef patties were formulated from beef lean, beef fat and hydrogenated soybean oil. Using the Pearson Square Method, the patties were formed to contain three total fat levels (16,20 and 24%). At each level, vegetable fat was substituted for 0,25 or 50% of total fat to end up with nine different blends. Aerobic psychrotrophs and mesophiles of the blends were investigated at 0-, 3- and 6-day intervals. A split-split plot design with two replications was used to differentiate significant and non-significant attributes of the main effect and the corresponding interactions. Results obtained show that total fat level, percentage vegetable fat or replication did not have a significant effect on bacterial numbers of ground beef extended with hydrogenated soybean oil of 70 iodine value. However, the days of storage significantly (P <0·001) affected both the Standard Plate Count (SPC) and Psychotrophic Plate Count (PPC). The log number of bacteria in both SPC and PPC continued to increase with length of storage. In all the cases studied PPC was found to be higher than SPC, which indicates that the predominant microflora in raw beef products is psychrotrophic in nature.