INTRODUCTION AND METHODS During an investigation of distribution and ecology of the Archiannelida of the estuarine area of the rivers Rhine, Meuse and Scheldt in the southwestern part of The Netherlands, three species of polychaetes new to the Dutch fauna were found. Because these species have not been reported very frequently from other localities, the finds are described in some detail. The first two species were found in samples taken with a o.i m2 Van Veengrab; the last one originated from a core sample taken in the beach. After collecting the samples have been treated in the way described by Boaden (1963). RESULTS AND DISCUSSION Hesionura augeneri (Friedrich, 1937) (fig. 2) This species was found at two localities in the coastal water (fig. 1) at depths of s and 8 m, respectively. The bottom consisted of medium sand. The salinity at these localities normally is over 16.5°/00 CI-. Description (fig. 2). — Length up to 20 mm =135 segments; diameter without parapodia up to 120 μ. Prostomium bluntly conical, but elongated in longer animals, distally with four antennae. Four small, sometimes barely visible eyes dorsally on the prostomium. First segment with one pair of ventral cirri, second segments with one pair of dorsal cirri as well. Parapodia of the third segment bearing setae and a ventral cirrus; all other parapodia with a dorsal cirrus as well. Ventral cirri longer than the dorsal ones; especially in larger specimens the ventral cirri have a deep brown colour. Parapodia uniramous with acicula and four setae, three being compound and one simple. Pygidium with two long cirri.