We report two patients undergoing continuous ambulatory peritoneal dialysis (CAPD) in whom peritonitis developed and nontuberculous mycobacteria were isolated from peritoneal fluid. In one, Mycobacterium avium-intracellularis was the only organism isolated. Despite a three-month course of antibiotics to which the organism showed in vitro sensitivity, there was no apparent response. The patient died, and an autopsy showed disseminated mycobacterial disease. In the second case, Mycobacterium fortuitum and diphtheroids were isolated from the peritoneal fluid. Although it was not clear that the mycobacterium was solely responsible for the peritonitis in the second case, the infection failed to resolve with antibiotic therapy appropriate for diphtheroids. This patient also died. Both patients had indolent, chronic infections, although there was granulocyte predominance in the peritoneal fluid. Both had involvement of the catheter exit site. To our knowledge, these are the first reported cases of nontuberculous mycobacterial peritonitis in CAPD patients. We recommend evaluation for mycobacteria, including cultures and stains of dialysate specimens, in all cases of CAPD-associated peritonitis where no organism is identified, or where no improvement is noted after 48 hours of therapy. Repeated cultures for mycobacteria are appropriate for suggestive cases. Since these infections are difficult to treat, it may be prudent to remove the dialysis catheter if they are isolated.