To provide insight into the mechanisms that enable sympatric-parapatric occurrence of multiple species of leaf beetles belonging to the genus Cassida Linnaeus (Coleoptera: Chrysomelidae: Cassidinae) in northern Japan, host specificity was estimated and mating behavior was observed in the laboratory among six species (Cassida japana Baly, Cassida nebulosa Linnaeus, Cassida rubiginosa Müller, Cassida vibex Linnaeus, Cassida fuscorufa Motschulsky, and Cassida viridis Linnaeus). Although behavioral isolation across the six species was notably incomplete, our data suggest that high host specificity will act as a strong habitat isolation barrier, thus preventing interspecific sexual interaction in the wild. The evolutionary processes that have led to sympatric-parapatric coexistence among Cassida beetles are also discussed.