The non-indigenous bivalve Theora lubrica has been intro­duced to the southwestern delta area of the Netherlands. For several years, it has been misidentified as Abra nitida, a species with similar appearance. Theora lubrica differs from A. nitida in the presence of an internal ridge (from umbo to anteroventral margin) and a bifid cardinal tooth in the left valve. In native and newly colonised areas the bivalve is particularly a successful competitor at sites with periodical hypoxic conditions. Ecology and invasiveness are discussed.