Three alien rotifers were found in the Schelde estuary in spring and summer 2002-2003. Lecane decipiens and Brachionus variabilis, with a maximum of respectively 6 and 4 ind L-1, were rare and quite localized in the freshwater tidal reach, corresponding to the stage III (non invasive species) in Colautti and Mac Isaac’s invasion terminology. This record of L. decipiens is the first confirmed one in Europe. Keratella tropica was generally present in the freshwater reach but, more tolerant to salinity, it was also found in brackish water. With its maximum abundance of 18 ind L-1, K. tropica appeared to be invasive, with a stage IVa (widespread but rare) at the scale of the Schelde estuary. A description with illustrations of these introduced species is given with some information about their ecology.