INTRODUCTION During a preliminary investigation in 1977 and the beginning of 1978 on the benthic infauna of the seagrass communities formed by Zostera marina L. and Zostera noltii Hornem, in The Netherlands, the polychaete Malacoceros fuliginosus (Claparède, 1869) was found. In Europe this spionid has been recorded earlier from Sweden (Hannerz, 1956), Denmark (Rasmussen, 1956, 1973), Germany (Westheide, 1966; Giere, 1968), England (Day, 1934), France (Bellan, 1964; Guérin, 1975) and The Netherlands (Wolff, 1973). In The Netherlands only two specimens had been found, viz. one in the mouth of the Oosterschelde and another one in the North Sea. During our investigation many animals were collected, mainly in Lake Grevelingen, a stagnant salt-water lake in the SW of The Netherlands, which was separated from the influences of the river Rhine in 1964 and from the North Sea in 1971. In the seagrass communities in the Oosterschelde no specimens and in the Dutch Wadden Sea two specimens have been found near the salt marsh "De Ans", south of the island Terschelling. This paper deals with the morphological characters of M. fuliginosus in Lake Grevelingen and provides some data on the ecology. METHODS The benthic infauna was collected with a sediment core (Ø 9 cm). The samples were taken randomly in a Zostera bed. The cores were driven to a depth of 10 cm below the sediment surface. Each sample was sieved through 0.5 mm mesh size and preserved in 70% alcohol. The four sampling sites were: 1. Lake Grevelingen (close to Herkingen). From May 1977 till April