Stable isotope and radiocarbon measurements clearly separate food crusts on Ertebølle pottery found at inland sites in the Trave and Alster valleys from food crusts on similar pottery from sites on the Baltic coast of Schleswig-Holstein. The only reasonable explanation of this pattern is that at the inland sites (and probably also at coastal sites) pottery was regularly, perhaps mainly, used to cook fish and other aquatic resources. Evidence from modern reference material shows that fish from the Trave and Alster Rivers are much more depleted in both 13C and 14C than marine fish, and that experimentally charred food crusts retain the isotopic signatures of the ingredients. We therefore contend that there is no scientific evidence that pottery was used in the Trave and Alster valleys before the mid-5th millennium cal BC, and that it is the frequency with which Ertebølle people cooked fish which creates the impression that pottery from inland sites is earlier than that from coastal sites.