1. Pear psyllids, belonging to the subgenenus Hepatopsylla, often exhibit strong seasonal dimorphism. The summer forms are lighter and smaller while the winter forms are relatively darker and larger. 2. We examined whether seasonal polymorphism in Cacopsylla bidens is a continuous or discontinuous phenomenon and assessed the role of temperature and photoperiod in its regulation. 3. Adult psyllids were sampled in pear orchards for three years using beating trays. The effects of temperature and photoperiod on induction of phenotyic changes were evaluated in the laboratory. Morphological parameters such as wing size and body coloration of both sexes were monitored. 4. Observation of the wild population indicated that C. bidens exhibited continuous phenotypic plasticity during the transition from summer to winter morphs but a step-wise transition from winter to summer morphs. 5. According to laboratory experiments both temperature and photoperiod play a significant role in the regulation of polymorphism in this species.