The development of stomata – the epidermal pores on plant leaves that regulate gas exchange – is tightly regulated by various environmental factors. Light, for example, promotes stomatal development; very few stomata are found on the epidermis of dark-grown seedlings. Here, on p. 3165, Ute Hoecker and colleagues report that auxin, acting via Aux/IAA proteins, plays a key role in repressing stomatal development in dark-grown seedlings. The researchers show that aux/iaa mutants, which display auxin insensitivity, exhibit excessive stomata production specifically in dark-grown seedlings. This stomata-overproducing phenotype is also observed in mutants that are defective in auxin biosynthesis or perception, suggesting that auxin acts to repress stomatal production in the dark. They could further show that the excessive formation of stomata is caused by an increase in cell divisions within the stomatal lineage. Finally, the researchers use a combination of epistasis studies to elucidate a genetic network that integrates light and auxin signals in order to regulate stomatal development.