The gene for ornithine decarboxylase in Saccharomyces cerevisiae, SPE1, has been assigned to chromosome XI by the technique of transverse alternating pulsed field electrophoresis and DNA-DNA hybridization. Genetic mapping by tetrad analysis shows that the SPE1 gene is located on the left arm of chromosome XI, 6 cM from the LAP1 gene and 43 cM from the TRP3 gene. The spe10 mutation previously isolated in this laboratory is mapped to the N-terminal region of the SPE1 gene, and therefore should be designated as a spe1 allele.