A 2-year-old boy with proven X-linked chronic granulomatous disease was placed under continuous co-trimoxazole prophylaxis. He remained free of infection for 4 years. At age 6.25 years, he suddenly developed a fever with no localizing signs and died 16 days later in septic shock. A gram-negative, catalase-positive, halophilic, aerobic bacterium was cultured from blood, bone marrow, and ascitic fluid. This organism could not be identified in microbiological laboratories in Europe and the United States. Its biochemical features indicate that it may belong to a species which has not previously been described.