The resistance of T. gambiense to pentavalent arsenicals, especially to tryparsamide, is not due only to the action of subcurative doses; it can exist as a natural characteristic of certain strains. This resistance is often difficult to create in the laboratory and the routine treatment given to patients does not inevitably increase it in the case of a relapse. If its frequency increases in an endemic area it is rather because the treatment has removed a large number of non-resistant strains from the stock of circulating trypanosomes. In old endemic areas there exist, alongside non-resistant strains which are generally easily transmitted, varieties of resistant ones which are sufficiently easily transmitted and which maintain the percentage of resistant cases; and finally, resistant strains which are hardly transmissible at all are found in chronic treated patients and relapsing cases.