The Tunicate Botryllus schlosseri (Pallas) was observed for the first time in the sluice-dock (Bassin de Chasse) of Ostend during the year 1960. This species was probably imported into Belgium together with spat from Zealand (Holland). During the year the species has spread and multiplied extensively. This species can therefore be considered as belonging to the Belgian marine fauna. The sexual reproduction and settling of new colonies took place during the months of August, September, October and November. During those months a settling of 98% was observed on the test surfaces in the immediate vicinity of the infected oysters. About 1 km from this locality in the Bassin de Chasse, where no infected oysters were found, a settling of only 0,66% was noted. This confirms laboratory tests concerning the short free living period of the larvae (on average only two hours). A preference for a determined depth could not be made out. The rapid growth and the extensive propagation in the for this animal favourable biotope forms a danger to the oyster culture. The overgrowing of the oyster suffocates and ultimately kills this mollusc, while as a food competitor, Botryllus could also be dangerous.