There is a worldwide concern about the expansion of non-indigenous species because they may threaten local biodiversity as some of them may successfully outcompete native species. We report on the presence of two introduced mud worm species, Boccardia proboscidea and Boccardiella hamata, in the southern North Sea, where the former species has been discovered on groynes along the Belgian coast and both species were detected among Pacific oysters Crassostrea gigas in the south-western Dutch delta. Boccardia proboscidea is native to the North Pacific and was first detected in Europe along the Spanish Basque coast in 1996. Our record is the third for European waters and the first for the North Sea. Although B. proboscidea occurs in limited numbers and in ephemeral habitats along the Belgian coast, the species probably has established a permanent population, since it has been reported repeatedly after its first discovery in January 2011. Boccardiella hamata is native to the Atlantic and Gulf coasts of North America. This species has never been recorded from Europe before.Our research demonstrates that in under-studied habitats, such as man-made hard substrata, even well-established populations of newly introduced species may stay undetected. Due to a lack of monitoring of these particular habitats and insufficient knowledge about introduced biota, introduced species may pass unnoticed or may be overlooked for several years. A dedicated monitoring will be necessary to detect introduced species in an early stage. In view of the upcoming Marine Strategy Framework Directive requirements such monitoring will be inevitable.