A gram-negative bacteria designated DF-3 was cultured on multiple occasions from stool samples of a patient with common variable hypogammaglobulinemia and chronic diarrhea. Antibiotic therapy resulted in elimination of the organism and resolution of the patient's symptoms. DF-3 has not been linked previously to human disease; because of its fastidious growth characteristics and unique isolation requirements, it may be a rarely identified cause of diarrhea and other gastrointestinal symptoms in immunocompromised patients.