Spore formation is a common process in the developmental cycle of fungi. In the yeast Saccharomyces cerevisiae, Ime2 is a key protein kinase for the meiotic cell cycle, which precedes ascospore formation. Here, we analysed the IME2-related imeB gene of the filamentous ascomycete Aspergillus nidulans. imeB deletion strains are retarded in growth and overproduce fertile sexual fruiting bodies in the presence of light, which normally represses sexual development. imeB mutants also display abnormal differentiation of sexual Hülle cells in submerged cultures. Increased sexual development of imeB mutants is dependent on VeA, a component of the heterotrimeric velvet complex. A combined deletion of imeB with the phytochrome fphA, a red light receptor, results in a complete loss of light response, suggesting that ImeB and FphA cooperate in light-mediated inhibition of sexual development. Furthermore, we found that imeB mutants fail to produce the mycotoxin sterigmatocystin, an aflatoxin precursor, and show that ImeB is needed for expression of the sterigmatocystin gene cluster. ImeB contains a TXY motif conserved in mitogen-activated protein kinases. This sequence element is essential for ImeB function. We conclude that ImeB is a mitogen-activated protein kinase-related protein kinase required for the co-ordinated control of light-dependent development with mycotoxin production.