The effect of pH and temperature on the growth and acidification characteristics of Streptococcus thermophilus 404 and Lactobacillus bulgaricus 398 were studied in order to optimize starter production. A quadratic two-variable model for each of these parameters is proposed. Optimal growth conditions were found at pH 6.5 and 40° C for S. thermophilus and pH 5.8 and 44° C for L. bulgaricus. Maximum acidification was obtained at pH values and temperatures higher than the optimal growth conditions. In addition, the two strains were generally more sensitive to pH effect.