The morphology and incidence of a spontaneous solid lymphoid tumour of Xenopus laevis was studied. 180 adult tumour-bearing animals were diagnosed in the course of observations made on a large laboratory colony over a period of 4 years. The annual incidence of the tumour varied between 1.5 and 7%. The disease was more prevalent in the laboratory stock than among newly imported animals. The tumours were found to have arisen chiefly in the liver, spleen and kidneys, and metastases to almost all other organs were observed. Electron microscopic studies showed some similarities between the tumour cells and the ‘monocyte-like’ cells encountered in the normal spleen. The examination of the malignant cells in the spleen and liver so far did not reveal the presence of virus particles.