A male red swordtail Xiphophorus hellerii with a large tumour in its branchial cavity was described in this study. The tumour, diagnosed as lymphosarcoma, grew rapidly, was pale yellow-white, hard in texture, and well-differentiated. It primarily infiltrated the gills and showed no visible metastasis. Histopathology of the tumour mass revealed dense sheets of tight compacted pleomorphic, round neoplastic cells with eosinophilic cytoplasm, exhibiting mild to moderate anisocytosis and anisokaryosis. Nuclei were generally round with fine granular chromatin. Mitotic figures were abundant. The precise aetiology of the tumour is unknown. The single occurrence of a tumour in this fish suggests that it developed spontaneously.