A random-bred CFW strain of mouse has proven to be very susceptible to a pathogenic strain of Trypanosoma cruzi "Tulahuen," of Chilean origin. The survival time, in days, for 8 to 10 g weanling mice, 12 to 15 g, 20 to 25 g, and 30+ g mice was determined after inoculation with 1000 to 5000, 10,000 to 30,000 and 40,000 to 50,000 organisms per g body weight. Regardless of weight or dose, mice survived for only 9 to 14 days. The greatest variation in survival was obtained with weanling mice. A total dose of 1000 to 5000 organisms per g body weight for 12 to 15 g mice was found to be satisfactory for maintaining stock infections in serial transfer every 7 to 10 days.