An adult female, entire domestic rat (Rattus norvegicus forma domestica) presented with swollen auricles and incoordination. The rat was diagnosed with bilateral otitis externa. Radiographs of the skull revealed bilateral otitis externa/media. Bacterial culture of a swap from the external ear canals identified large numbers of Pasteurella pneumotropica as well as fewer Streptococcus merionis, Bacteroides fragilis and Fusobacterium spp. Microscopic examination of a deep skin scraping and a core needle biopsy of the pinnae diagnosed lymphoma. The animal was euthanased after being treated unsuccessfully symptomatically for 10 days. Postmortem examination revealed a cutaneous T-cell lymphoma, mycosis fungoides, associated with bilateral otitis externa/media and metastases into several lymph nodes, thymus and lungs. Cutaneous T-cell lymphoma has not been reported in pet rats to date. Lymphoma should be considered as a differential diagnosis in rats presented with swelling of the pinnae.