The largest Catholic dioceses in the United States are Los Angeles, California; New York City, New York; Chicago, Illinois; and Boston, Massachusetts. A diocese is a defined territory overseen by a bishop. An archdiocese, which is a territory with a very large Catholic population, usually exists in large metropolitan areas.
Brazil claims the largest number of Catholics, with 145,446,000 members in 268 dioceses as of 2004, constituting about 79 percent of the country's population. Mexico has 123,393,000 Catholic members in 90 diocese, or about 87 percent of the country's population. Mauritania and Algeria have the fewest number of Catholics, with 4,000 in Mauritania and 3,000 in Algeria. Mauritania has only one diocese, while Algeria has four.