The publication by ENGEL, GEERTS and VAN REGTEREN ALTENA (1940) on Alderia modesta (Loven, 1844) and Limapontia depressa Alder & Hancock, 1862, in the estuaries of southwestern Holland (provinces of Zuid-Holland and Zeeland) induced us to look for these animals on other Dutch mud-flats, viz. in the Waddenzee, where they had not been collected before. First we inspected the gullies between the mud flats, which contain more than 1 metre of water at high tide and, in addition, the Zostera nana-zone, which is dry at low tide and about under 50 cm of water at high tide. Lateron we searched for algae in the brackish inland waters, which will be mentioned below. In all these localities we did not find a single Alderia modesta or Limapontia depressa.