The ¹³C content of 42 human beings from Denmark and Greenland, ranging in age from about 5500 BC to AD 1750, has been measured to elucidate their food habits. The measurements have shown that the reliance on sea food relative to terrestrial food by prehistoric man in Denmark has varied greatly in different archaeological periods, as reflected by a range of delta ¹³C values from -11.4% to -26.6%. These differences far exceed what had been estimated from the archaeological record itself. Due to differences in the ¹⁴C content of terrestrial and of marine food, these variations render ¹⁴C ages of human beings somewhat uncertain. In general, an evaluation of the food supply of human beings is a prerequisite for precise estimates of their age from ¹⁴C determinations.