Some recent models for transmission dynamics of malaria and Japanese encephalitis have incorporated non-random selection of host individuals by vector mosquitoes or selection of alternative host species according to relative host density and attractiveness. Effects of these factors on transmission dynamics were evaluated in terms of the basic reproductive rate of pathogens (R) and infection rates of host and vector populations. Dye and Hasibeder (1986) and Hasibeder and Dye (1988) suggested that non-homogeneous contact between vector and host populations increases R and contributes to persistence of malaria infection, although the proportion of infected humans does not always increase with increased non-homogeneity. Kingsolver's model (Kingsolver, 1987) showed that preference of infected hosts to healthy hosts in vector mosquitoes also contributes to malaria persistence. A simple model for malaria control by use of bednets was presented here, which incorporated blood-seeking behavior among humans using and not using bednets. If mosquitoes attack unprotected people more frequently than before the use of bednets in the area, malaria control may be difficult because R can increase due to introduced non-homogeneity in vector-human contact. Sota and Mogi (1989a, b) used a model system of one vector species with two bloodmeal host species to analyze variable influences of livestock density on malaria and Japanese encephalitis dynamics. They suggested that zooprophylaxis for malaria control may fail when introduced livestock provide more bloodmeals for mosquitoes and contribute to an increase in mosquito density. In their model for Japanese encephalitis, relative density of humans to swine and host preference in mosquitoes determine the risk of human infection. The model suggested that the human risk is at maximum when mosquitoes feed equally on humans and swine, and that a high human risk is present even at low swine densities because of strong preference for swine in mosquitoes.