In summer 2006 an extensive mortality of Pacific oysters Crassostrea gigas occurred in Lake Grevelingen, the Netherlands. A sample of Pacific oysters was investigated for the presence of shellfish pathogens as potential causes of the mortality. Yellow-green lesions were observed in several oysters upon clinical inspection. Histopathology showed that 6 out of 36 oysters had a suspected bacterial infection, including 4 Nocardia-like infections. Two bacterial species, Vibrio aestuarianus and Nocardia crassostreae, were isolated from haemolymph samples and identified using PCR and sequencing of the 16S rRNA gene. This is the first isolation of N. crassostreae from shellfish in European waters. The near full-length 16S rRNA sequence of this Dutch Nocardia sp. isolate was identical to other known N. crassostreae isolates from the west coast of North America. The primary cause of oyster mortality was thought to be the physiological stress from environmental conditions, including prolonged high water temperatures and low oxygen levels. The multiple bacterial species isolated from the diseased Pacific oysters may have been a secondary cause.