ABSTRACTThree arguments are presented to defend the use of the nameAgaricus brunnescensinstead ofA. bisporusfor the cultivatedAgaricus: 1) The namesA. brunnescens(described in 1900) andA. bisporus(described in 1926) were originally applied to the same species; 2) a fresh collection recently made in Toronto, Ontario, is conspecific withA. brunnescens; 3) the Toronto collection is genetically and taxonomically inseparable from the cultivatedAgaricus.Arguments are supported by evidence from the literature, examination of authentic herbarium material and by nuclear and mitochondrial DNA restriction fragment analyses.