Distinguishing zebra mussels from quagga mussels suddenly became an issue in The Netherlands after the first observation of the latter one in 2006 in the Hollands Diep. For most of the Dutch malacologists the quagga mussel is an unknown nonindigenous species in contrast to the well-known naturalized zebra mussel. Based on shell morphometry a distinction is possible in most cases by checking the ventral margin of the valves, which is somewhat curved in the quagga mussel (fig. 4). If the difference with the nearly straight ventral margin of zebra mussel valves is unclear the lateral cross section of both species should be taken into account. In zebra mussels the transition between the dorsal and ventral side of the valves forms a definite angle, a carina, which is rounded in quagga mussels (fig. 3). If both characteristics do not allow to distinguish between both species, the diameter/height coefficient could be taken into account which is in general <1 in quagga mussels and >1 in zebra mussels with shell lengths > 14 mm (fig. 1). However, for zebra mussels it was found that this metric varies with the area where the species had grown (fig. 2).