p-Rosaniline-sensitive and -resistant strains of Trypanosoma gambiense were separately passaged in mice treated with furazolidon. After 10 serial passages in mice, the trypanosomes were found to be resistant to furazolidon when tested in mice. The rate of appearance of AK forms after injection of furazolidon into mice infected with a clone obtained from the p-rosaniline resistant strain by repeated treatment with furazolidon (WRF) was lower than that in mice infected with a clone obtained from the p-rosaniline sensitive strain by the same treatment with furazolidon (WSF). Moreover, WSF was resistant to p-rosaniline, although it had not been exposed to it. These results indicate that there is an interaction between the actions of furazolidon and p-rosaniline on the kinetoplast. The existence of this interaction was supported by electron microscopic observations on the kinetoplasts of trypanosomes after the injection with p-rosaniline and furazolidon into mice infected with the p-rosaniline resistant clones, WSF and WRF.