Type 2 diabetes (T2D) is more prevalent among African-American (AA) than European-American (EA) women for reasons that are unknown. Ethnic differences in physiological processes related to insulin sensitivity (SI) and secretion, and age-related changes in these processes, may play a role. The purpose of this study was to identify ethnicity- and age-related differences in SI and β-cell responsivity among AA and EA females, and to determine whether these differences are independent of body composition and fat distribution. Healthy, normoglycemic females aged 7-12 years (n = 62), 18-32 years (n = 57), and 40-70 years (n = 49) were recruited for entry into this study. Following an overnight fast, SI, intravenous glucose tolerance (Kg), acute C-peptide secretion (X0), and basal, first-phase, second-phase, and total β-cell responsivity to glucose (PhiB, Phi1, Phi2, and Phi TOT, respectively) were measured by an intravenous glucose tolerance test. Total % body fat was assessed by dual-energy X-ray absorptiometry, and intra-abdominal adiposity (IAAT) by computed tomography. Main effects of age group and ethnicity were measured with analysis of covariance, adjusting for % fat, IAAT, and SI as indicated. AA had lower SI, and higher Kg, X0, Phi1, and PhiTOT (P < 0.05), which remained after adjustment for % fat and IAAT. Greater X0, Phi1, and PhiTOT among AA were independent of SI. Advancing age was associated with greater Phi2 among both EA and AA. To conclude, inherent ethnic differences in β-cell function exist independently of adiposity and SI. Future research should examine whether ethnic differences in β-cell physiology contribute to disparities in T2D risk. © 2010 The Obesity Society.