Nine male and five female adult free-living platypuses, obtained in a prospective capture-release study from northern Tasmania, exhibited gross features of cutaneous mycosis caused by Mucor amphibiorum. The lesions were present on the hind limbs (six cases), from limbs (four), tail (five), dorsal trunk (three) and ventral trunk (one). They varied in size, and ranged from raised red nodules or plaques, which sometimes exuded purulent material, to ulcerated lesions with central cavitation, red exuding centres and raised epidermal margins. Older lesions were covered either partly or fully by thickened and irregular epidermis. Histological examination of skin biopsies revealed discrete, poorly encapsulated granulomas, or more commonly a diffuse granulomatous or pyogranulomatous inflammation. Inflammatory cells consisted of neutrophils or eosinophils, sparse plasma cells and lymphocytes, many macrophages and occasional multinucleated giant cells. Fibrovascular tissue was diffusely and irregularly scattered in the granulomatous regions. Sphaerules characteristic of M. amphibiorum infection were observed in all lesions. The cutaneous distribution of the lesions and the natural history of the platypus indicated that entry of M. amphibiorum may have been via superficial skin wounds. T cells were the predominant infiltrating lymphoid cells in the diffuse lesions, indicating the importance of the cell-mediated response to infection. (C) 2000 Harcourt Publishers Ltd.