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A large species diversity has recently been discovered in the genus Agaricus. Six subgenera and 23 sections are now recognised. In this study, three specimens collected from Thailand, formed a monophyletic clade in subgenus Pseudochitonia, based on analyses of ITS sequence data. Further analyses, based on multi-gene sequence data (ITS, LSU, tef1-α), using BEAST, revealed that this clade originated 26.7 Ma. According to their distinct morphological characteristics, phylogenetic position and relatively old divergence time, a new section Cymbiformes is proposed and this section is represented by a new species A. angusticystidiatus. This new section is characterised by the strong iodoform odour of basidiomes and cymbiform basidiospores. Descriptions, colour photographs and illustrations are presented.