In the Text
Cite the U.S. Census Bureau as the author, either in the text of a sentence, a parenthetical citation or a footnote.
Specify the year of the census -- in this case, 2000. If desired, you can include the exact name: United States Census 2000.
Note the relevant page number, if you discuss specific information.
In the Bibliography
Write "U.S. Census Bureau" as the author, and alphabetize the entry under "U."
Write the exact title of the document you reference. Unfortunately, you cannot simply mention a fact concluded by the census; you must cite a specific document that includes the fact. Perhaps the most comprehensive document would be "U.S. Summary: 2000, Census US Profile"; however, many other official government documents present information about the 2000 census, such as the "Introduction to Census 2000 Data," the official Census 2000 website, and various summaries, briefs, profiles and special reports. You could also cite the actual census questionnaires used to obtain the data.
Cite the publication date of the document you select. Check the document itself rather than making an assumption and writing "2000"; for example, the government did not publish the "U.S. Summary: 2000" until 2002.
Specify your chosen source's medium of publication. For example, if you retrieved information from the Census Bureau's website, then cite "Web" as the medium. The Bureau also offers census information in print form and on CDs.
Note the publisher's location--Washington, D.C. (Since you already cited the U.S. Census Bureau as the author, you don't need to repeat it as the publisher.)