Mitogen activated protein kinase cascades function in eukaryotic responses to the environment and stress. Trypanosomatid parasites possess protein kinases with sequences characteristic of kinases in such cascades. In this report we use gene knockouts to demonstrate that two mitogen activated kinase kinase genes, MKK1 (Tb927.3.4860) and MKK5 (Tb927.10.5270), are not essential in the pathogenic bloodstream stage of Trypanosoma brucei, either in vitro or in vivo. Bloodstream forms lacking MKK1 showed decreased growth at 39 degrees C as compared to the parental line. However, unlike its Leishmania orthologue, T. brucei MKK1 does not appear to play a significant role in flagellar biogenesis.