ABSTRACT.Photoperiod significantly affected size‐related aspects of metamorphosis in the milkweed leaf beetleLabidomera clivicollis(Kirby) (Chrysomelidae). Larvae reared under short‐day photoperiod successfully pupated at lower weights than larvae reared under long‐day photoperiods. This short‐day effect was observed to an equivalent extent in both males and females; but within photoperiod treatments, males consistently pupated at lower weights than females. It is likely that the short‐day effect on metamorphosis is associated with photo‐periodically induced changes in larval corpora allata activity or juvenile hormone titre. Ecologically, the short‐day effect on metamorphosis may enhance the moulting success of larvae which encounter food shortages