The Schizosaccharomyces pombe cdc37 gene has been identified and was found to be essential for cell viability. To further study S. pombe Cdc37, a range of mutants were generated using both random and directed mutagenesis. Cdc37 temperature-sensitive mutant cells at the restrictive temperature stop dividing within a single cell cycle. Cdc37 protein levels were not changed at the non-permissive temperature in mutants, indicating that cell inviability arises from defective function of the mutant Cdc37 protein. Morphologically, temperature-sensitive mutant cells arrest with a uniform phenotype, being elongated, characteristic of the cdc phenotype. About 80% of these elongated cells contained a single nucleus with a 2C DNA content, indicating that cdc37 temperature-sensitive mutants arrest in G2. The only exception was cdc37-J, where half the cells leaked through to mitosis after 8 hours at the restrictive temperature and displayed divided nuclei and septa, arresting with defects in cytokinesis. Characterisation of the cdc37 temperature-sensitive mutants led to the identification of Cdc2 as a client protein and a principal candidate for the cause of the G2 cell cycle arrest. Cdc2 activity was dramatically reduced within one hour at the non-permissive temperature in cdc37 temperature-sensitive mutants, but Cdc2 protein levels remained constant. Biochemical and genetic interactions between Cdc37 and Cdc2 were observed, supporting the idea of Cdc2 as a client of Cdc37. Cdc37 is believed to deliver client protein kinases to the co-chaperone Hsp90, forming a heterocomplex. A small fraction of total cellular Cdc37 in s. pombe forms a high molecular weight complex which also contains Hsp90, Cdc37 was found to biochemically and genetically interact with Hsp90, indicating these co-chaperones co-operate in S. pombe.