The polo-like protein kinase gene family (PLKs) encodes proteins which are involved in the control of exit from mitosis in higher eukaryotes. We have cloned and analysed a polo-like kinase, tbplk, from an evolutionary divergent eukaryote, Trypanosoma brucei. The gene encodes a 767 amino acid protein of predicted size 86.8 kDa with 50.4% identity to mammalian PLKs over the protein kinase catalytic domain and it possesses a conserved motif, the 'polo-box', which is found in all PLKs. Phylogenetic analysis demonstrates that this gene is clearly a member of the PLK family, although it has some distinctive features such as a large C-terminal insertion when compared with mammalian PLKs. The gene is single copy and expressed in both bloodstream and procyclic stage trypanosomes. Sequencing of tbplk from a number of trypanosome isolates reveals a length polymorphism in a run of asparagine residues within the coding region. The presence of PLKs in a wide range of organisms, including such a primitive organism as T. brucei, suggests that PLKs may have a key role in the function of the cell cycle.