Lacazia loboi (syn. Loboa loboi), the etiological agent of lobomycosis, was compared in human and dolphin tissue using light and electron microscopy, and computer-assisted morphometrics. The histological features of the lesions were similar; however, preliminary electron microscopy data indicates that cell wall destruction may vary in the two hosts. Calcofluor stained tissue sections of human and dolphin tissue were examined with UV light microscopy and the images digitized. Measurements of area, minimum and maximum diameters, and perimeter were made. Student's t-test (alpha = 0.01) revealed that L. loboi cells infecting dolphin tissue were significantly smaller than those infecting human tissue. This study represents the first comparative analysis of the morphology of the etiological agent of this disease in its two known natural hosts. The data indicate that the organism may not be identical in the two hosts.