The mushroom, Agaricus bisporus (Lange) Imbach, has been in cultivation since 1650 and is now a major protected crop with a value world-wide in excess of £2,OOOm. Despite its importance as a crop species and long history of cultivation, little work has been done on genetics and strain improvement. The mushroom is 2-spored and single-spore cultures are usually fertile. Studies of the breeding system of the mushroom are described based on the behaviour of cultures derived from aberrant 3- and 4-spored basidia. It is shown to be a secondarily homothallic species with a single mating-type factor. Methionine and proline auxotrophs are identified and two morphological markers are characterised, the first resulting in the constitutive production of fruit-body initials in culture and the second in altered gill morphology. Studies of 4-spored wild Agaricus species are described. A. bitorguis, ~ macrosporus and A. nivescens are shown to be unifactorially heterothallic. Nuclear numbers in homokaryons and heterokaryons of these species are determined and compared with A. bisporus. A scheme is proposed for the evolution of the secondary homothallism seen in ~ bisporus from a 4-spored heterothallic ancestor. Approaches to mushroom strain improvement are considered in detail and a breeding strategy based on the use of induced fungicide resistance is described. Following this strategy mutants resistant to the fungicides carboxin, benodanil and tridemorph have been obtained and evaluated in vitro and in vivo. Four of these mutants are the subject of a patent under the aegis of the British Technology Group. Finally, the mechanism regulating secondary homothallism has been studied. A predictive hypothesis, the random migration of nuclei in pairs from the basidium into the spores, has been tested in the model 2-spored organism Coprinus bilanatus nom. provo