The growth and viability of Bifidobacterium lactis and Lactobacillus acidophilus per se and for both strains as a coculture were studied in reconstituted skim milk so as to assess their dependence upon temperature (5, I0 and 15C), NaCl concentration (0, 0.51 and 1.03 mol L-'), and storage time (0-8 weeks). Cultures of B. l a d s exhibited no signijkant loss of viability either with increasingNaCl concentration or increasing storage temperature within the ranges studied On the other hand, viability of L. acidophilus decreased with increasing temperature and increasing NaCl concentration under similar conditions. When cocultured with L. acidophilus, B. lactis was signiJicantly less tolerant to higher NaCl levels and higher temperatures than when in pure culture, although $rial numbers were still above the threshold required for commercial application. Coculturing with B. lactis had no detrimental effect on viability of L. acidophilus. A mechanistic model, which considers the behavior of the pure and mixed microbial populations is described by specijic death rates which vary with temperature following Arrhenius relationships) and NaCl levels following simple inhibition kinetics) in the milk medium. Activation energies for the death rates of B. lactis and L. acidophilus were 14 kcal mot ' and 9-15 kcal mol-I, respectively.