AbstractThe name Isoetes setacea has been applied to two different species, causing nomenclatural and taxonomic confusion. Some authors have rejected it on that account, yet it is currently used in a sense that is incompatible with its type. That type represents one of two similar species, I. echinospora and I. lacustris, which still today grow together at the type locality. An epitype is designated here, ensuring that I. setacea is sunk in synonymy under I. lacustris. A second name potentially threatening I. echinospora is I. tenella. It is currently regarded as a synonym of I. lacustris, but its type, a plate in Flora Danica, cannot be confidently identified, nor are there any specimens associated with it. Again, an epitype is designated to cement the current disposition of I. tenella and remove that second threat from I. echinospora. The correct name of the species generally misnamed I. setacea is I. delilei, here lectotypified.