Trypanosoma musculi brought into Plasmodium berghei-infected mice in the later stages of the malaria infection shows rapid, approximately logarithmic multiplication in the peripheral blood. The trypanosome number increases by a factor of 2-9 per day, multiplication of the parasites kills most mice in a few days. Many multiplicative forms of the trypanosomes and a few trypanosomes without nuclei are seen in blood smears. Histologically and in touch preparations, masses of multiplicative forms of the trypanosomes are seen in the sinusoids of the liver. Aggregates of trypanosomes are found, too, in the kidney medulla and, inconstantly, in venous sinuses of the spleen and postcapillary venules of lymph nodes. Occasionally, trypanosomes may be found extravascularly in the interstitial connective tissue especially of the pancreas, in lymph vessels and lymph node sinuses. - Mice infected in earlier stages of the Plasmodium berghei infection achieve stabilization of trypanosome numbers.