SINCE the original work by Maire1, the attention of a number of cytologists2–4 has been attracted to the study of nuclear behaviour and basidiospore formation in the two-spored cultivated mushroom, Agaricus campestris Fr. var. bisporus5. Sass2 first published a full account of meiosis in the basidium, and although in one of his diagrams eight bivalents are clearly figured he reported a diploid chromosome number of approximately eight, a determination which was later confirmed4. In the interim, however, Colson3, after a most comprehensive study, reported a chromosome number of n = 9.