During paleoparasitological analyses on several Neolithic sites in Switzerland (Arbon-Bleiche 3) and southwestern Germany (Hornstaad-Hörnle I, Torwiesen II, and Seekirch-Stockwiesen), numerous eggs of Diphyllobothrium sp. were recovered. This is one of the earliest occurrences of this parasite during the prehistoric period in the Old World. The prevalence of this helminth in the samples studied raises the question as to how important parasitic diseases were during the Neolithic period and what their actual consequences were.