Soluble esterases prepared from 10 strains of the house fly were separated into 24 electrophoretic bands by Ogita''s thin layer electrophoretic technique. These esterase bands were distinguished by electrophoretic mobilities. They were classified into enzymatic families on their behavior differences. Linkage tests showed that one of the bands might be controlled by a genetic factor on the 4th chromosome, and the others by genetic factors on the 5th chromosome. They were named as Est-A and Est-B loci. A recombination value between the cm and Est-A loci was ca. 20.7[plus or minus], and that between the cm and Est-B loci was 50.6 [plus or minus] map units. It was indicated that the esterase bands active upon naphthylacetate esters were not correlated with the diazinon-resistance gene.