An atypical sucrose-fermenting Corynebacterium diphtheriae strain was isolated from three blood cultures of a 14-year-old girl presented to a university teaching hospital in Rio de Janeiro, Brazil. She had mitral endocarditis that proved to be fatal despite intensive antibiotic therapy. Blood cultures showed a fluorescent Gram-positive, aerobic, coryneform-like bacillus presenting pyrazinamidase and CAMP reaction negative. The isolate was identified as a toxigenic strain of C. diphtheriae var. mitis by both Elek and radial immunodiffusion (RID) tests. The invasive C. diphtheriae sucrose-fermenting biotype strain adhered to glass surfaces and expressed pronounced hemagglutinating activity (titer 8), a property common among the nonfermenting biotype strains. Laboratories should be alert to the possibility of the isolation of C. diphtheriae with a positive sucrose fermentation test, especially when nontoxigenic strains are isolated from uncommon anatomic sites.