Genetics of host–related Feeding and ovipositional behaviors of the Colorado potato beetle,Leptinotarsa decemlineata(Say), were described, comparing a population from Morelos, Mexico, and a population from Rhode Island, United States. Traits studied were larval feeding acceptance, oviposition ability, and oviposition preference. A preliminary attempt to select for potato intolerance in larval feeding in the Morelos population reduced feeding acceptance of potato after three generations. Host-related difference between the populations in oviposition ability could be explained by inheritance of as few as one gene or two genes with epistasis, suggesting that major genetic change in at least one locus was involved in the original host expansion ofL.decemlineatafrom native hosts to potato. Significant dominance by genome from the Rhode Island population was expressed by hybrids in both feeding acceptance and oviposition preference. Results were used to develop a working hypothesis to explain genetic mechanisms responsible for the original host expansion ofL.decemlineatato potato.