The authors underline the frequency with which trichomoniasis is located in the prostate. In these cases the parasite is rarely in its trophozoite form but has a circular shape which resembles the resistant pseudocystic type. They are found by phase contrast microscopy after staining with 1% cresyl blue which makes the difference with white blood cells. Out of a total of 178 men presenting with urethroprostatic signs (urethral discharge and burning), sterility with oligospermia, or a history of dyspareunia, the circular type of trichomoniasis was found in 40 (22%) of the cases, with urethritis in 26 cases, and prostatitis in the other 14 patients. Two courses of treatment, at one month's interval, with Tinidazole (2 tablets daily for 6 days) or Nimorazole (2 tablets daily for 8 days) produced healing in 4 cases with urethroprostatic effections, disappearance of dyspareunia in 2 out of the 3 cases treated, and return of fertility in 3 out of 7 patients. An up-to-date list of references is given.