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Preliminary incubation of raw milk samples to evaluate bacterial quality has been a commonly recommended practice. The presumption has been that selective growth of a segment of the microflora associated with unsanitary methods and conditions would be enhanced. The present work involved a theoretical approach to evaluate this presumption. Pure cultures of Pseudomonas aeruginosa, P. fluorescens, and Escherichia coli as well as enriched mixed cultures of psychrotrophs and mesophiles were used. The generation time of each culture was determined at various temperatures. An analysis of covariance showed the coefficients of generation time related to temperature were not significantly different. Thus, preliminary incubation in the range of the temperature now recommended does not have a selective effect for specific groups of microorganisms.