This study is the first assessment of non-indigenous ascidians (NIA) in Newfoundland. Field work was conducted from 2006-2007 in four harbours to assess the abundance and biodiversity of megainvertebrates on wharf pilings, including indigenous and non-indigenous ascidians. Quadrat samples, visual surveys and photographic records were taken in each harbour. The most common species found in the survey were Mytilus spp. Two NIA were also found, Botrylloides violaceus and Botryllus schlosseri. -- Variation in cytochrome oxidase I gene of mitochondrial DNA was analyzed for these NIA, as well as for two indigenous ascidians (Boltenia echinata & Halocynthia pyriformis), in order to determine within and between species variation for future use in genetic marker design and to identify probable source populations. There was less nucleotide dissimilarity within species (≤15.6 %) than among species (17.7 - 25.8 %). The probable source populations of B. schlosseri in Newfoundland are from locations in the Northwestern Atlantic and Europe, specifically the Atlantic and Mediterranean Sea coasts of France and Spain
