Components of mitogen-activated signal transduction pathways have been shown to be involved in flagellum biogenesis and maintenance. A mitogen-activated protein kinase homologue, designated LmxMPK9 from Leishmania mexicana, has been recently identified in a homology screen and its mRNA found to be present in all life stages. Three different splice-addition sites were used for mRNA maturation in trans-splicing in the different life stages. However, here we show that LmxMPK9 protein is exclusively found in the promastigote stage. Recombinant expression of LmxMPK9 in Escherichia coli and kinase assays revealed a temperature optimum at 27 degrees C, the optimal growth temperature for L. mexicana promastigotes, and a preference for manganese to promote substrate phosphorylation of myelin basic protein. A deletion mutant for the single-copy gene revealed significantly elongated flagella, whereas overexpression led to a subpopulation with rather short to no flagella suggesting a role for LmxMPK9 in flagellar morphogenesis.