Ascomycetes are the largest group of true fungi, and characteristically produce their sexual spores in a sac-like structure called the ascus. They include medicinal agents (such as ergot), plant pathogens (Dutch elm disease is caused by an ascomycete) and yeasts used in fermentation. We have found the oldest ascomycetous fungi with flask-shaped ascocarps in thin-section preparations of the Lower Devonian (400 million years old) Rhynie chert of Aberdeenshire, Scotland1. This discovery has implications for dating the origin of this group of fungi, and underscores the diversity of fungal-plant interactions early in the colonization of the land.