We report the first use of 14C isotope analysis to investigate the ecological grouping of earthworms. Mature endogeic (Allolobophora caliginosa), mature epigeic (Lumbricus rubellus), and semimature anecic worms (A. longa) were collected in September 2002 from a woodland site at Lancaster, UK. Because anecic worms are known to have a variable feeding behaviour and can show dietary changes during ontogeny, additional immature and mature specimens of A. longa were also collected from the same site in January 2004. Epigeic earthworms showed the lowest radiocarbon concentration (0–3-years old), implying that they assimilated more recently fixed carbon than the anecic or endogeic earthworms. The age of carbon assimilated in mature anecic species (5–7-years old) was closer to that of endogeic species (5–8-years old) than to epigeics, suggesting that a greater proportion of older, more mineralised organic matter may form part of the diet of the anecic earthworms than previously thought. These results suggest that 14C approaches are useful in the study of the feeding behaviour of detritivorous animals by providing in situ information on the age of the carbon assimilated by the worms. This can then be related to their role in ecosystem functioning, particularly in carbon cycling.