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SUMMARYAn understanding of sexuality and life cycle has been conspicuously lacking for members of the genus Agaricus despite the prime importance of Agaricus bisporus to the edible mushroom industry. Recent work on fertility patterns among monosporous isolates of rare 4-spored basidia suggests secondary homothallism and a bipolar pattern of sexuality as characteristic of this species. A full elucidation of sexuality and its relation to the life cycle, however, depends upon the use of genetic techniques. Nutritionally deficient mutations have accordingly been obtained in several strains. Nuclei bearing these genetic markers were traced in matings and demonstrated to be associated in balanced heterokaryons. The markers were also shown to segregate and recombine in the first generation of monosporous isolates from fruiting bodies. The analysis provides proof of heterokaryosis and strong evidence for haploidy of vegetative nuclei, a sexual cycle consisting of nuclear fusion and meiosis, and the usual inclusion of two meiotic nuclei in each basidiospore. Main features of the life cycle have been clarified, and the case for bipolarity and secondary homothallism in the species has been strengthened. Possible procedures for interbreeding different strains of A. bisporus are discussed.