Using a proteomics approach, we identified 13 differentially abundant proteins in heads of nondiapause, early, and mid-diapause Culex pipiens L. adult females. Three proteins that were more abundant during early diapause showed highest identities to pupal cuticle protein (PCP). The majority of less abundant proteins were involved in energy metabolism and cytoskeletonal reorganization. The increase of PCP in early diapause may be involved in enhancing stress resistance in the cuticle, a response akin to the general enhancement of stress resistance associated with overwintering insects. PCP also may have utility as a biomarker for early diapause in this species.