The existence of four alleles of phosphoglucomutase (PGM1) in human red cell lysates has previously been demonstrated by isoelectric focusing (Bark et al., 1976; Kühnl et al., 1977; Sutton and Burgess, 1978). Experiments are now described in which the position of each of the first-locus (PGM1) and second-locus (PGM2) isoenzymes is defined, thus extending and confirming the original proposal made by Bark et al.