In order to determine the colonisation patterns of several Mycoplasma species in homosexual men, urethral, oral and rectal specimens from 10 homosexual men with acute non-gonococcal urethritis (NGU) and 18 without NGU were examined using sensitive methods. Mycoplasma hominis and Ureaplasma urealyticum existed in both groups, which is in keeping with previous studies of heterosexual men. Mycoplasma genitalium was detected in the rectum of both NGU-positive and NGU-negative men and in the urethra of one man with chlamydia-negative NGU, but not in those without urethritis. Mycoplasma fermentans was found in the throat and rectum only and Mycoplasma penetrans in all three anatomical sites. In contrast, Mycoplasma pirum was found in the rectum only, that is, in 5 of the 28 men studied. Infrequent examination of this site is a possible explanation for previous failures to detect Mycoplasma pirum at a mucosal surface.