Field tests were conducted to study the effects of heterosis on mating competitiveness ( c ) of chemosterilized males of Aedes aegypti (L). When males of the normal strain were sterilized by exposing pupae to P,P-bis(I-aziridinyl)-N-methylphosphinothioic amide and released along with untreated males and virgin females of the same strain, c averaged 97%. When hybrid males replaced either the sterile or untreated normal males in releases designed to measure the effect of heterosis, no discernible effect was noted, whether the hybrids were sterile (c = 97%) or fertile (c = 109%). The results indicate that chemosterilized males may be suitable for genetic control of this species, but that hybrid vigor does not necessarily increase the competitiveness of the released males.