Although the prostate gland is believed to serve as a parasite reservoir in trichomoniasis in men, and clinical association of trichomonads with prostatitis is common, there has been, to our knowledge, no unequivocal demonstration of Trichomonas vaginalis within the prostate gland. Using established immunoperoxidase procedures, we have positively identified trichomonads in the prostatic urethra, glandular lumina, submucosa, and stroma. Foci of nonspecific acute and chronic inflammation, as well as intraepithelial vacuolization, were associated with the infection. The finding of trichomonads within and beneath glandular epithelium necessitates reevaluation of the traditional view of T vaginalis as a strictly surface-dwelling organism.