A survey of the isoenzyme patterns of phosphoglucomutase (PGM) as demonstrated by starch gel electrophoresis has been undertaken in different species. Seven mammals, two birds, one reptile, two amphibians, four fish, and four invertebrates were studied, and in some cases several tissues were examined. In all cases the predominant enzyme present was a group of electrophoretically related enzymes which are believed to all represent expression of the PGM1 allele. In some species, examples of other groups of isoenzymes were encountered, presumably representing other genetic loci, corresponding to PGM2 and PGM3. These were always less in amount. The PGM from the chicken was unique in the survey in that its mobility changed with storage. A somewhat similar although not identical change could be produced by addition of PCMB to the partially purified enzyme. Both altered enzymes, i.e., that resulting from storage and that produced by addition of PCMB, were more heat labile.