The pathogenicity to mice of a stock of Trypanosoma evansi from Mauritania has been compared to that of stocks from Kenya, Niger, Chad and China, using doses of 5.10(5), 2.10(6) and 5.10(6) trypanosomes per mouse. The survival of mice inoculated with stocks from Kenya, Niger and China is short, on average 2.1 to 6.5 days. Mice inoculated with stocks from Mauritania and Chad survive longer, 12.5 to 22.7 days, depending on the dose.