In recent years, the actin cytoskeleton in Schizosaccharomyces pombe has become the subject of intense scrutiny. However, to date, only a single actin mutation has been identified. Described here is the isolation and characterization of four new cold-sensitive actin mutations. Sequence analysis of the mutant actin genes indicated that each of these mutations caused alterations in single amino acids that are conserved in all actin sequences. These mutants differ in their phenotypes. One of these mutations (act1-48) was identified as an extragenic suppressor of a mutation in the cdc4 gene, which is required for actin ring formation and cytokinesis. Interestingly, when act1-48 mutant cells were shifted to the restrictive temperature, actin patches were not detected but the actin ring formation and stability was unaffected. The three other mutations, act1-16, act1-32 and act1-67, primarily affected the actin ring formation or stability while F-actin patches did not seem to be substantially different in appearance. Given that the ultrastructural architectures of F-actin patches and the F-actin ring are presently unclear, these mutations, which affect one structure or the other, should be useful for future studies on the role of actin itself in the function of these F-actin-containing structures in S. pombe.