Background: Vaginal epithelium is the most important site for initial contact of the parasite infection in humans. Objective: To investigate the effect of Lactobacillus acidophilus (LA) and pH on attachment of T. vaginalis isolated from symptomatic and asymptomatic patients to vaginal epithelial cells (VECs). Methods: Following 1-4 hrs contact of parasite with VECs, wet mounts were prepared and used for measuring cell adherence in vitro. This was done by addition of whole LA, its excretory secretory product (ESP) and changing the pH of the media either by HCl or LA. The mean of the three readings of the experiments was taken to compare the isolates and their controls. Results: The exposure of VECs to whole LA for one hour led to enhancement of adhesion and then a gradual fall with ultimately non-viability T.vaginalis after 4 hrs of incubation with T. vaginalis. In the presence of ESP a gradual decreased of adhesion was observed. The number of VECs attached by T.vaginalis was higher in symptomatic than asymptomatic women’s isolates when pH was reduced to 4.5 by HCl, when pH was reduced to a similar level by LA this ratio was 47% and 35% respectively. Conclusion: The presence of LA enhanced the attachment of parasite at initial steps of infection, but its effect was deleterious thereafter. This study should be paralleled by appropriate in vivo experiments and if substantiated can be used for effective therapy and prevention of human trichomoniasis.