Evidence that chemical changes in litter exposed to elevated CO2 might alter the composition and function of fungal communities in soil is presented. Some potential effects of elevated atmospheric CO2 on the decomposition of Fraxinus excelsior leaf litter and the interactions of the colonizing fungi, growing singly or in simple associations in microcosms, were investigated. Fungal colonization was monitored by analysis of the ergosterol content of litter and specific PCR-amplified ribosomal DNA spacer products. After < 42 d, fungal colonization was less on litter with a higher C[ratio ]N ratio when obtained from seedlings grown in elevated CO2 (600 ppm). After triple-species inoculation percentage α-cellulose and, in some combinations, nitrogen content was reduced on litter from seedlings grown in elevated CO2.