Reinforcement of pre–zygotic isolation can result when any of several kinds of selection act against hybrids. This paper investigates the situation where hybrids are selected against for ecological reasons, for example when there is no niche for individuals that are phenotypically intermediate between the parental species. The calculations here show how much ecological selection can lead to the reinforcement of a female mating preference or an assortative mating trait that is expressed in both sexes. The model allows for the ecological trait to be affected by any number of loci, but assumes that selection is weak and the introgression rate small. The effect of selection against hybrids increases rapidly as the difference between the mean phenotypes of the two populations increases. When genetic variation in the ecological trait is caused by many loci, stabilizing selection on it further contributes to reinforcement.