Inoculation of Escherichia coli (serotype O:6) into the bladder of male and female Mastomys (Praomys) natalensis produced severe prostatitis. In this rodent both male and female animals possess a well developed prostatic gland. The histologic and microbiologic course of the prostatic infection resembled strongly the human disease. Acute bacterial prostatitis was followed by the development of chronic bacterial or nonbacterial prostatitis. The infection persisted in some animals for up to six months. Prostatitis was observed histologically in all animals sacrificed six months postinfection. Animals responded to the infection with a rise of anti-lipopolysaccharide antibodies. No major morphologic differences were detected in the histologic pattern of the inflammatory process between animals with positive and negative bacterial cultures and between male and female animals.