Clinical and morphologic findings are described in a 62 year old woman with "mixed connective tissue disease" who received corticosteroid therapy, and in whom disseminated and fatal Petriellidium boydii infection with right-sided endocarditis developed. The patient was a gardener. The organism is ubiquitous in soil in many part of the United States; therefore, it is likely that the infection was introduced by this means. Endocarditis due to P. boydii has not been reported previously. In the patient described, massive vegetations nearly obliterated the tricuspid valve orifice, encasing a pacemaker catheter which had been inserted eight years earlier. Although never previously isolated from blood cultures, P. boydii was isolated from 11 consecutive blood cultures. P. boydii is a true fungus and has only recently been appreciated as an opportunistic pathogen in a compromised host.