In 1955, a causeway was built across the Strait of Canso, Nova Scotia, Canada, preventing in the Strait a free interaction between the Gulf of St. Lawrence and Atlantic waters. In order to evaluate a possible impact of this barrier on the marine environment, the distribution of foraminifera in sediments was investigated.Data were collected from early May to late August, 1973. During this period, the water on the Gulf of St. Lawrence side of the causeway was colder in early spring, but warmer and less saline during the summer as compared to the Atlantic side.At the causeway, the surface 1–3 cm of the bottom sediment consists of very soft and black to dark brown mud on both sides. On the Atlantic side, these fine sediments cover bedrock; but on the Gulf of St. Lawrence side, they cover sands and gravels.The 76 foraminiferal species collected in surface samples were subjected to cluster analysis, which defined two distinct groups of stations separating the fauna on the two sides-of the causeway. The characteristic species of the Gulf of St. Lawrence side is Ammonia beccarii. In the subsurface layers the dominance of this species decreases within a zone extending 4 km to the north of the causeway. On this evidence it was concluded that prior to the causeway, the Atlantic waters extended at least 4 km further to the north.