A large number of molds serve as producer strains for the industrial production of pharmaceuticals, foods, or organic chemicals. To optimize strains for production processes, conventional strain development programs use random mutagenesis and, more recently, recombinant technologies to generate microbial strains with novel and advantageous properties. The recent detection of mating type genes in fungal production strains and the discovery of cryptic sexuality in presumably asexual fungi open up novel strategies for generating progeny with new, as yet unobserved properties. Mating type genes, which can be considered as "sex genes," not only direct sexual development but also regulate a broad range of fungal secondary metabolites. In addition, they control hyphal morphology, which has a direct impact on production processes that are often conducted in huge fermenter tanks. Here, we survey the occurrence and function of mating type genes that have been discovered in a wide range of industrial fungal producer strains. The possibility to obtain progeny from industrial producers by sexual mating provides an exciting alternative to conventional strain improvement programs aiming to generate optimized recombinant production strains.