We describe the case of a 2-year-old boy with disseminated infection by a rapidly growing, poorly pathogenic mycobacterial species that belonged to the Mycobacterium fortuitum-Mycobacterium peregrinum complex. He had a severe course characterized by a poor response to treatment and recurrent lymph node abscess formation. Sequencing of the interferon-gamma receptor 1 gene (IFNgammaR1) revealed that he was homozygous for a novel null mutation, 453delT. Patients presenting with disseminated infections by rapidly growing environmental mycobacteria must be investigated for complete IFNgammaR1 deficiency. The spectrum of IFNgammaR1 genotypes associated with this immunological disorder is expanding.