A six year old male, Aruba Island rattlesnake, Crotalus unicolor, with diffuse cranial swelling and anorexia was diagnosed with multicentric lymphosarcoma with lymphoid leukemia. Significant hematologic findings included a marked leucocytosis (137,000), monocytosis and basophilia with marked heteropenia and mature lymphopenia. The predominant (95.5%) white cells were large blast-like cells. Cytological examination of a tissue aspirate from the head revealed a highly cellular monomorphic population of round mononuclear cells. Histologic examination revealed that most parenchymal organs and tissues contained monotonous sheets of neoplastic lymphoid cells.