We asked whether ambient temperatures can affect morph frequencies within a subarctic population of thepolymorphic leaf beetle Chrysomela lapponica through thermal melanism and/or developmental plasticity. Bodytemperature increased faster in beetles of dark morph than in beetles of light morph under exposure to artificialirradiation. Dark males ran faster than light males in both field and laboratory experiments, and this differencedecreased with increasing ambient air temperature, from significant at 10 °C to non-significant at 20 °C and26 °C. On cold days (6–14 °C), significantly more dark males than light males were found on their host plants incopula (40.8% and 27.3% respectively); on warm days (15–22 °C) this difference disappeared. Light femalesproduced twice as many eggs as dark females; this difference did not depend on the ambient temperature. Theproportion of dark morphs in the progenies of pairs with one dark parent was twice as high as that in theprogenies of pairs in which both parents were light, and this proportion was greater when larvae developed atlow (10 and 15 °C) than at high (20 and 25 °C) temperatures. We conclude that low temperatures may increasethe frequencies of dark morphs in C. lapponica populations due to both the mating advantages of dark males overlight males and developmental plasticity. Variation in frequencies of low-fecund dark morphs in the population,caused by among-year differences in temperature together with density-dependent selection, may contribute tothe evolutionary dynamics of the colour polymorphism and may influence abundance fluctuations in these leafbeetle populations.