Laboratory-reared Aedes aegypti were selected for blood-feeding efficiency. Feeding speed and host irritating properties were used as selection criteria. Mice and man respectively served as mosquito hosts. Bidirectional selections during 14 generations did not result in changes in feeding speed in either selection line. It was concluded that the heritability of variations in feeding speed was either very low or absent in these mosquitoes. This implied that the bimodality of the distribution of the variations in feeding speed observed had to be attributed to pheno-typical factors. Selections by the repeated test method for 4 generations did not result in changes in the host irritating properties of the mosquitoes, nor were these properties found to be a stable characteristic of the individual mosquito. It consequently appeared that differences in host irritating properties were not genetically determined either. Variations in host-seeking behavior were found to be affected by multiple factor inheritance.