The author of this article describes residential segregation of foreigners in Barcelona. She presents some theoretical considerations, makes statistical calculations pertaining to the problem, and defines causal factors of residential segregation. What is important, residential segregation in Barcelona may be far deeper than in other cities because of the relatively strong separatism in Catalonia and the required knowledge of the Catalan language. These factors make full access to local labour market very difficult for immigrants and lead to their marginalization. In the statistical calculations a measure of residential segregation (Duncan, Duncan 1955) is used which allows a comparison of the situation in Barcelona and in other cities, as well as an analysis of changes in Barcelona in recent years.