Only one documented case of a malignant cartilaginous tumor has been previously reported from fish, a large cranial chondrosarcoma from the paddlefish, Polyodon spathula (Walbaum). Described here are two chondrosarcomas diagnosed from adult laboratory-reared Rivulus marmoratus Poey, the first reports of the tumor type from an atheriniform fish. Histologically, the tumors consisted of cartilage in various stages of differentiation. These ranged from poorly differentiated areas consisting of a highly cellular, primitive mesenchymal component with numerous mitotic figures and extensive anaplasia to well-differentiated portions expressed as scattered foci of mature cartilage consisting of single and multiple chondrocytes isolated within lacunae. The lesions, possibly arising from the sclera, were invasive into the eye, with elements of the choroid and striated muscle trapped within the tumor matrix.