Recent work with a mouse model of Trichomonas vaginalis infection indicated that estrogenized BALB/c mice that were preinfected with Lactobacillus acidophilus showed a greater duration of T. vaginalis infection as compared to a control group of mice that were not treated with L. acidophilus. To examine the interaction between T. vaginalis and L. acidophilus further we performed in vitro competitive growth assays between the 2 species. Although the addition of L. acidophilus to the T. vaginalis cultures slowed the growth of the protozoa, the added bacteria did not increase trichomonad death. However, T. vaginalis had a deleterious effect on L. acidophilus growth in combined cultures when compared to matched controls. Using an initial inoculum of 10⁵/ml, at 40 hr the control L. acidophilus concentrations had grown to 1.3× 10⁷/ml. However, in combined culture with T. vaginalis, L. acidophilus concentrations at 40 hr had fallen to 7.8× 10⁵/ml and 6.1× 10⁴ for the 10:1 (T. vaginalis at 10⁴) and 1:1 (T. vaginalis at 10⁵) test ratios, respectively (P < 0.01). This demonstrates that T. vaginalis can cause the concentration of L. acidophilus to fall in vitro and may explain why the concentration of L. acidophilus in the vagina falls in trichomoniasis.