Sowerby described Agaricus cossus in 1799. The fungus possessed a smell, resembling that of a wounded larva of Cossus cossus (Lepidoptera). The species belongs in Hygrophorus, and since more than one white Hygrophorus species has this distinctive smell the epithet cossus has been variously interpreted. The complete internal transcribed spacer (ITS) region of the original type collection made in 1794, preserved in the Royal Botanic Gardens Kew herbarium, was successfully sequenced. Comparison with the ITS sequences from four other white aromatic-acidulous smelling Hygrophorus species, including the type specimen of H. quercetorum, showed that H. cossus is a species associated with Quercus and an older name for H. quercetorum. The differences in basidiome colouration developing with age and host-tree association appear to be the most useful characters to discriminate between the four species with a Cossus cossus smell. A table of morphological and ecological characters is provided.