Defects in the Schizosaccharomyces pombe (Sp) cell cycle-controlling genes prevent the cell cycle progression. Mutations in one of the late septation genes, cdc4, cause Sp cells to arrest at cytokinesis and result in an elongated cellular morphology. By functional complementation of one of the temperature-sensitive (ts) mutant alleles of cdc4, cdc4-31, with a Sp cDNA library, a novel gene, scf1, which suppresses the elongated ts phenotype of cdc4-31, was isolated. DNA sequence analysis of the cDNA revealed homology with a small heat-shock protein family, HSP12, of Saccharomyces cerevisiae (Sc). Expression of this gene is highly induced, both at 37 degrees C and in the stationary phase of cell growth. It is likely that scf1 is expressed in stress conditions such as heat-shock or nutritional limitation. The phenotypic suppression of cdc4-31 by a small HSP12 homolog, Scf1, suggests that the functional loss of cdc4, which is involved in formation of the F-actin contractile ring, can be prevented or repaired by one of the small HSP. This implies that an HSP might be involved in late cell plate formation or in stabilization of the Sp F-actin contractile ring structure.