Ecological speciation studies have more thoroughly addressed premating than postmating reproductive isolation. This study examines multiple postmating barriers between host forms of Neochlamisus bebbianae leaf beetles that specialize on Acer versus Salix trees. We demonstrate cryptic isolation and reduced hybrid fitness via controlled matings of these host forms. These findings reveal host-associated postmating isolation, although a non-ecological, ‘intrinsic’ basis for these patterns cannot be ruled out. Host preference and performance results among cross types further suggest sex-linked maternal effects on these traits, while family effects indicate their genetic basis and associated variation. Genes of major effect appear to influence these traits. Together with previous findings of premating isolation and adaptive differentiation in sympatry, our results meet many assumptions of ‘speciation with gene flow’ models. Here, such gene flow is likely asymmetric, with consequences for the dynamics of future ecological divergence and potential ecological speciation of these host forms.