Blastomycosis cannot yet be prevented or controlled, in part because the natural habitat of the causative fungus, Blastomyces dermatitidis, remains ill defined. In investigating 2 outbreaks of blastomycosis that occurred in the summer of 1985 among persons engaged in activities along rivers in contiguous central Wisconsin counties, we isolated B. dermatitidis from soil at one of the riverbanks. Blastomycosis developed in 7 (58%) of 12 residents and guests who has gathered at a pheasant farm on the Tomorrow River in early May, and in 7 (88%) of 8 boys and 1 adult who had visited a site on the Crystal River in early June. Of the 14 patients, 13 (93%) were symptomatic. Two patients visiting the sites only once became ill 23 and 78 days after exposure, respectively. We traced one outbreak to fishing from the bank of the Tomorrow River, and the other to climbing into an underground timber fort along the Crystal River. A culture of soil and ogranic debris from the fishing site yielded B. dermatitidis. From these and other outbreaks, and studies of endemic disease, we conclude that riverbanks can be a natural habitat of B. dermatitidis, and that the environment around waterways represents the most important site yet identified for transmission of B. dermatitidis.