The published literature on serum selenium levels in the US population describes studies on small samples that may not be representative of the US population. This analysis provides the first nationally representative serum selenium levels in the US population by age group, sex, race-ethnicity, poverty income ratio (PIR), geographic region, and urban status. The Third National Health and Nutrition Examination Survey (NHANES III) is a national population-based cross-sectional survey with an in-person interview and serum selenium measurements. For the 18,597 persons for whom serum selenium values wereavailable in NHANES III, the mean concentration was 1.58 nmol/L and the median concentration was 1.56 nmol/L. Mean serum selenium levels differed by age group, sex, race ethnicity, PIR, and geographic region. The US population has slight differences in serum selenium levels by demographic factors.