A previously selected Plasmodium-refractory strain of Anopheles gambiae melanotically encapsulates many species of Plasmodium. Genetic studies of this strain have shown that this refractory phenotype is controlled by a limited number of genes, and the existence of two such genes, Pif-B and Pif-C, has been demonstrated. Further work to determine the molecular basis for this mode of refractoriness led to the discovery that the host-parasite interaction is mimicked by the mosquito's response to carboxymethyl (CM)-Sephadex beads injected into the thorax. These small beads are melanized within 24 hr in refractory mosquitoes but are rarely melanized in susceptible ones. Because of the considerable potential in using bead melanization as a model for Plasmodium refractoriness, we performed a genetic analysis of the differential response to beads. Reciprocal crosses of susceptible (4arr) and refractory (L35) mosquitoes and an analysis of F1 phenotypes were done. The F1 progeny had a phenotype similar to that of the parental refractory mosquitoes; therefore, dominant refractory allele(s) must be present in the refractory strain. Males from the reciprocal crosses had identical phenotypes, indicating that X-linked loci did not have a visible effect on the melanizing phenotype. To further study the mode of inheritance of the melanizing trait, a backcross of F1 females to 4arr males was done. The phenotypic distribution of the backcross progeny was bimodal, and the melanization phenotypes were similar to those of the susceptible and refractory parents. These data suggest that a small number of loci are responsible for the differential response to CM-Sephadex beads, and that one chromosomal region contributes strongly to the melanizing trait. Because the 4arr strain carries mutations in the pink eye (X) and red eye (III) genes, possible linkage of the melanizing phenotype to these two genes was tested. No linkage with either marker was detected. The pattern of inheritance of the melanizing phenotype is similar to that of the refractory phenotype of Pif-B; therefore, the genetic basis of the two responses may be the same.