Aspergillus flavus is the major producer of carcinogenic aflatoxins in crops worldwide and is also an important opportunistic human pathogen in aspergillosis. The sexual state of this heterothallic fungus is described from crosses between strains of the opposite mating type. Sexual reproduction occurred between sexually compatible strains belonging to different vegetative compatibility groups. Multiple, indehiscent ascocarps containing asci and ascospores formed within the pseudoparenchymatous matrix of stromata, which places the fungus in genus Petromyces. The teleomorph of P. flavus could not be distinguished from that of P. parasiticus (anamorph = A. parasiticus), another aflatoxin-producing species, based on morphology of the sexual structures. The two species can be separated by anamorph morphology, mycotoxin profile and molecular characters.