Lactobacillus casei IFO 3012 and Kluyveromyces fragilis(KFCC 35458) were cultured together in Soymilk to investigate the growth characteristics and the conditions suitable for acid Production. L. casei produced more amount of acid rapidly when cultured with K. fragilis in soymilk than when cultured singly. The optimum conditions for acid production by the mixed cultures of L. casei and K. fragilis were achieved with a temperature of $35-37^{circ}C$, a 1:5-1:9(O.D 660) ratio of L. casei to K. fragilis at inoculum, a 1.0 level of sucrose fortification or a 2.0% level of skim milk powder fortification and a culture time of 24hr. Under these conditions the amounts of acid produced by the single culture of L. casei and the mixed cultures with K. fragilis were 0.31% and 0.44% in soymilk, 0.43% and 0.97%, respectively, in soymilk fortified with 1.0% level of sucrose. These indicate that the amount of acid produced by mixed cultures is about 1.42 fold greater in soymilk and about 2.26 fold greater in soymilk fortified with 1.0% level of sucrose than that produced by the single culture of L. casei. The amount of acid produced in soymilk fortified with 2.0% level of skim milk powder was 1.0 level for both of the single culture of L. casei and the mixed cultures of L. casei and K. fragilis after 24hr incubation. In soymilk fortified with skim milk power less than 1.5 the mixed culture with K. fragilis showed higher content of acid than the single culture of L. casei only.