A case of authochthonous mycetoma with Petriellidium boydii as the causal agent, located in the third distal of the lower left leg is reported. The patient was a 35 year old male without any associated pathology. It is assumed that the infecting organism entered the tissues through a lesion caused a few years previously during a traffic accident. On admittance patient was found to have two fistulas on the upper part of the external malleolus from which pus with small yellow granules measuring 1 to 1.5 mm in diameter discharged. Fungus was repeatedly isolated in pure culture in all media in which is was sown. Mycological study of the isolated fungus and histopathology of the lesion are presented. The fungus proved in vitro to be sensitive to miconazole nitrate with minimal inhibitory concentration and minimal fungicide concentration values of less than 0.1 microgram/ml. The third distal of the peroneal bone was excised and arthrodesis of the ankle practiced as conservative treatment and miconazole administered intravenously, Eighteen months later clinical evolution is satisfactory.