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Ri, a measure of commonality of surnames in two populations, is compared with Rw, a measure of common ancestry from pedigree analysis. The data are derived from surname lists and from pedigree information for 380 married individuals living in the eight church districts of the Nebraska Amish of Pennsylvania. The low correlation of the two measures, and the conflicting results when comparing within-district and between-district relatedness using each measure, indicate that the two indices probably are measuring different things. Caution must be exercised in using a measure of isonymy to gauge relatedness in a population with few common surnames, and a long history of close inbreeding.