BACKGROUND: Diphtheria, a life-threatening respiratory disease, is caused mainly by toxin-producing strains of Corynebacterium diphtheriae, while nontoxigenic Corynebacteria, such as C. pseudodiphtheriticum rarely causes diphtheria-like illness. Recently several global diphtheria outbreaks have resulted from the breakdown of healthcare infrastructures particularly in countries experiencing political conflict. This report summarizes a laboratory and epidemiological investigation of a diphtheria outbreak among Forcibly Displaced Myanmar Nationals in Bangladesh. METHODS: Specimens and clinical information were collected from patients presenting at Diphtheria Treatment Centers. Swabs were tested for toxin-gene (tox) bearing C. diphtheriae by real-time (RT) PCR and culture. The isolation of another Corynebacterium species prompted further laboratory investigation. RESULTS: Among 382 patients; 153 (40%) tested tox-positive for C. diphtheriae by RT-PCR; 31 (20%) PCR-positive swabs were culture-confirmed. RT-PCR revealed 78% (298/382) of patients tested positive for C. pseudodiphtheriticum. Of patients positive for only C. diphtheriae, 63% (17/27) had severe disease compared to 55% (69/126) positive for both Corynebacterium species, and 38% (66/172) for only C. pseudodiphtheriticum. CONCLUSIONS: We report the confirmation of a diphtheria outbreak and identification of a co-circulating Corynebacterium species. The high proportion of C. pseudodiphtheriticum co-detection may explain why many suspected patients testing negative for C. diphtheriae presented with diphtheria-like symptoms.