Diapause is a halt in development that allows insects to survive the unfavorable conditions of winter. While the ability to enter a photoperiodically-induced diapause is a critical adaptation for many insects occupying temperate climates, the molecular basis of this trait is not well understood. In temperate populations of the Asian tiger mosquito, Aedes albopictus, pupae and adult females exposed to short day lengths produce diapausing eggs. Tropical populations of A. albopictus do not undergo a photoperiodic diapause. In order to investigate the molecular underpinnings of photoperiodic diapause in A. albopictus, we performed suppressive subtractive hybridization (SSH) of RNA transcripts from the mature (stage IV) follicles of females from a temperate population exposed to short-day (8L:16D) vs. long-day (17L:7D) conditions. The resulting cDNA library was sequenced and approximately 45 expressed sequence tags (ESTs) were characterized. In order to identify which of the corresponding 45 genes are involved in photoperiodic diapause, quantitative RT-PCR was performed on RNA extracted from follicle tissue for each of three temperate (diapausing) and two tropical (nondiapausing) populations exposed to short-day and long-day light regimes. Analysis of ESTs overexpressed in short-day vs. long-day samples in replicate temperate populations but not in replicate tropical populations resulted in the detection of genes associated with photoperiodic diapause in A. albopictus.