Females and eggs of Toxorhynchites r. rutilus were labeled with 32P by feeding fourth-stage larvae 32P labeled Aedes aegypti larvae. Eggs from females up to 3 weeks in age had detectable levels of radioactivity and individual eggs contained ca. 0.3% of the mother's total radioactivity. Comparisons of labeled and unlabeled females in indoor and outdoor cage tests indicated that survival and fecundity of the 2 groups were approximately equal. No differences were noted for dispersal and fecundity of labeled and control females released in field tests. The 32P-labeled Tx. r. rutilus females behave similarly to unlabeled females, and this method of radiolabeling provides a sound tool for tracking laboratory-reared females released into an area with an indigenous population.