A taxonomic and nomenclatural study of Psathyrella lepidotoides and P. populina, two species frequently confused in the European mycological literature, is performed. According to their morphological, molecular, and ecological characteristics, we conclude that both species are clearly distinguised and that, in addition, they should be included in the genus Cystoagaricus, in which we propose the pertinent combinations. We designate a lectotype for Agaricus populinus. Finally, based on the various interpretations by the authors over time, we consider that Cystoagaricus sylvestris (≡ Psathyrella sylvestris) is an ambiguous name, which does not represent either of the two previously mentioned taxa and, therefore, its earlier neotypification and synonymy are rejected. We propose the replacement of the neotype by a specimen of P. cotonea that is not in conflict with the protologue, which implies a new synonymy of P. sylvestris with P. cotonea, without negative consequences from the nomenclatural point of view