The non-sporulating diploid strain V327 of Saccharomyces cerevisiae was previously isolated in a search for thermosensitive autolytic mutants. This strain is very efficient at releasing intracellular proteins into the medium when incubated at high temperatures. The expression of this lytic phenotype depends on a morphogenetic defect, consisting of the appearance of elongated chains of cells. Transmission electron microscopy revealed a mislocalization of septa at semi-permissive temperatures and a total lack of septation together with abnormal cell wall architecture at a non-permissive temperature. The septin-encoding CDC10 gene was cloned by complementation of the pleiotropic phenotype of the V327 mutant. Rescue and sequencing of CDC10 alleles from V327 revealed a point mutation that created a single amino acid change in a region which is well conserved among septins. This new allele was named cdc10-11. The construction of a cdc10-11 haploid strain by substituting the CDC10 gene with the rescued allele permitted further genetic analyses of the mutation and allowed the construction of new homozygous cdc10-11 diploid strains that showed a reduced ability to sporulate. Fusing both the wild-type and the cdc10-11 alleles to green fluorescent protein (GFP) demonstrated that the mutation does not affect the localization of this septin to the bud neck at the standard growth temperature of 24 degrees C, although the morphogenetic phenotype at 37 degrees C parallels the disappearance of Cdc10-GFP at the ring encircling the septum area.