Homogenates of single individuals of two natural populations and five laboratory populations of Culex pipiens were examined by combining electrophoresis and heat denaturation studies on phosphoglucomutase (PGM). All populations showed a high degree of polymorphism for isoelectrophoretic temperature-resistant (tr) and temperature-sensitive (ts) alleles. Formal genetic data on the heat stability differences of the PGM are given. If both electrophoretic and isoelectrophoretic alleles are taken into account, the mean increase in the degree of heterozygosity is quite remarkable, i.e., about 65%.--The data are considered in relation to the biological significance that this new type of variability of structural genes could have in natural populations.