1. For the first time, Crepidula fornicata was observed in Belgium on 28-9-1911; in 1959, it became an oyster pest. 2. It was first introduced into Belgium with seed oysters from Great Britain. Later on, with seed oysters from the Netherlands. 3. Probably, the geographical spreading is caused by the adult animals attached to oysters. Larval propagation is responsible for local extension. 4. In the sluice-dock at Ostend, larvae occur from the middle of May till the end of November; they swim freely for a period of about 12 days. The first massive emission of larvae is observed during May and June, a second one in September. 5. The larvae settle preferentially at a depth of 65 cm. 6. Maximum settling takes place in May. It then decreases till October. 7. After two months, the animals already form chains consisting on an average of 4 specimens. 8. Maximum growth takes place from May until the end of August. After a lapse of five months, a length of 24.1 mm was recorded. 9. Only those individuals which settled in May and June persist in relatively large numbers. 10. Four-month o1d Crepidula were found incubating eggs. The number of eggs averaged 1,500.