The life cycle of African trypanosomes is characterized by the alternation of proliferative and quiescent stages but the molecular details of this process remain unknown. Here, we describe a new cytoplasmic protein kinase from Trypanosoma brucei, termed TBPK50, that belongs to a family of protein kinases involved in the regulation of the cell cycle, cell shape and proliferation. TBPK50 is expressed only in proliferative forms but is totally absent in quiescent cells despite the fact that the gene is constitutively transcribed at the same level throughout the life cycle. It is probable that TBPK50 has very specific substrate requirements as it was unable to transphosphorylate a range of classical phosphoacceptor substrates in vitro, although an autophosphorylation activity was readily detectable in the same assays. Complementation studies using a fission yeast mutant demonstrated that TBPK50 is a functional homologue of Orb6, a protein kinase involved in the regulation of cellular morphology and cell cycle progression in yeast. These results link the expression of TBPK50 and the growth status of trypanosomes and support the view that this protein kinase is likely to be involved in the control of life cycle progression and cell division of these parasites.