The effect of reduction in water activity on growth and acid production by Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris and Lactobacillus casei was studied at water activity values of 0.998, 0.975, 0.967 and 0.959. Reduced water activity resulted in extended lag time of varied magnitude, varied growth rates, different cell population and acid production. The highest growth rates were observed at an a_w of 0.998 (control). Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris and Lactobacillus casei reached a maximum cell population of 80x10⁶ CFU/ml, 60x10⁶ CFU/ml and 60x10⁶ CFU/ml after 24^th hour of incubation respectively.At water activity value of 0.975, Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris and Lactobacillus casei reached a maximum cell population of 40x10⁵ CFU/ml, 51x10⁵ CFU/ml and 51x10⁵ CFU/ml respectively after 24^th hour of incubation , which indicate that Lactococcus lactis subsp lactis  reached a maximum cell population which is lower than that observed for the other two strains. Which indicate that for each bacterial strain there is an optimum water activity at which the growth rate is faster comparing with the growth rate at lower or higher water activity. In addition the sensitivity to culture water activity varied between different bacterial strains. Also the obtained results indicated that the value of culture water activity had approximately simlar effect on rate of increase in developed titratable acidity as that found for the rate of increase in colony forming units.