Human phosphoserine phosphatase (E.C. 3.1.3.3) (PSP), a phosphohydrolase, participates in the biosynthesis of serine from carbohydrates (Moro-Furlani et al., 1980). Starch gel electrophoresis has identified isoenzymes and rare allelic variants. More common electrophoretic variation is thought to be due to secondary nongenetic modification. Using starch gel electrophoresis of interspecific somatic cell hybrids, we have been able to assign the gene for this enzyme to human chromosome 7.