Cell cycle control by cdc2-related kinases (CRKs) is essential to the regulation of cell proliferation and developmental processes in many organisms. Alternating phases of growth, arrest, and differentiation are characteristics of the infectious cycle of many trypanosomatid parasites, raising the possibility that members of the trypanosomatid CRK gene family participate in the regulation of these essential processes. Here we describe properties of the CRK3 gene from Leishmania major, which encodes a 36 kDa protein kinase showing 60% amino acid sequence identity with human CDK2, including several conserved sites implicated in regulation of kinase activity. CRK3 mRNA was constitutively expressed throughout the parasite life cycle, but histone H1 kinase activity of an epitope tagged CRK3 protein was greater in log-phase than in stationary-phase promastigotes. When integrated into the genome and expressed at the optimal level, CRK3 was able to rescue the growth defect of a Schizosaccharomyces pombe cdc2 mutant (cdc2-33(ts)), indicating that CRK3 is a functional homolog of cdc2. Mutants of CRK3 at several key regulatory residues showed the expected dominant negative effects on the S. pombe mutant. This is the first example of functional expression of a trypanosomatid CRK in yeast, opening the way for further genetic studies within this amenable organism.