Working with two strains of A. stephensi, respectively selected, for susceptibility and insusceptibility to Plasmodium gallinaceum infection, intraspecific polymorphism and intrastrain monomorphism have been noted. In fact, individuals at the fourth larval stage belonging to the susceptible strain possess an electrophoretic band, having esterase activity (Est 1), which migrates anodically, more slowly than Est 1 of the insusceptible larvae. Individuals representing the F₁ offspring obtained by crossing the susceptible females to the insusceptible males, and viceversa, possess both the parental enzymes. In the F₂ generation, individuals having the slower enzyme, others with both the parental bands and finally others possessing the faster enzyme have been obtained, according to the ratio 1 : 2 : 1. These data suggest that the two different molecular forms of Est 1 are phenotypic expressions of the activity of two autosomal loci, which may be written Est 1^s/Est 1^f/ and Est 1^s/Est 1^f, for the susceptible and unsusceptible strains, respectively.