Agaricus (Basidiomycota) is a genus of saprobic fungi that includes edible cultivated species such as Agaricus bisporus, the button mushroom.There has been considerable ecological, nutritional and medicinal interest in the genus, yet its biodiversity remains poorly known, particularly in subtropical and tropical areas.Classification of the tropical taxa has for a large part followed the classification of temperate species.The objective of our study was to examine to what extent this system of classification is appropriate for tropical Agaricus species.Species from temperate sections were therefore compared to the major clades of tropical taxa using a phylogenetic approach.ITS1+2 sequence data from 128 taxa were used in the phylogenetic analysis.Specimens included four species of genera closely related to Agaricus, 37 temperate species representing the eight classical sections of the genus, and 87 putative species of Agaricus from tropical areas of Africa, Asia and America.Bayesian and maximum likelihood analyses produced relatively congruent trees and almost identical clades.Our data showed that (i) only about one-third of tropical taxa belong to the classical sections; the systematics of the genus therefore needs to be reconsidered; (ii) basal clades and branches contain only African and Asian tropical taxa suggesting that the genus originated from these areas; (iii) among the remaining two-thirds of tropical taxa, those from America and those from Africa and/or Asia grouped in distinct clades, suggesting that secondary diversification occurred in these two areas and that direct migration between these areas has never or rarely occurred; (iv) in contrast, several clades of classical sections contain American and African+Asian species along with temperate species.These two conclusions added to the fact that the distribution range of some temperate species extends to both Europe and North America, suggest that the genus would have migrated from tropical Africa +Asia to tropical America over land via non-tropical regions, rather than by air or via tropical land masses.Such a scenario would have required climatic adaptation of the species and connexion between America and Eurasia.In this study, we collected approximately 50 distinct species from a small area of northern Thailand, mostly being probably novel species.This remarkable diversity indicates that Agaricus is a species rich genus in the tropics as well as in temperate regions and species numbers are grossly underestimated.