This report is one in a series that compares data from the American Community Survey (ACS) with data from the 2000 Census of Population and Housing. This report compares national estimates of ancestry, including type of report, such as single, multiple, unclassified, or no ancestry reported. It also analyzes specific groups reported, such as German or Irish. The Census 2000 Supplementary Survey (C2SS) was a nationally representative sample within the ACS program. This analysis compares C2SS and Census 2000 distributions, highlights differences that are both statistically and substantively different, and suggests possible explanations. The paper will also compare the 2005 ACS ancestry data with the C2SS data on ancestry.