Possible interactions between mycorrhiza, atmospheric CO2, free-living soil microorganisms and protozoa were investigated in pot experimental systems. Pea plants (Pisum sativum L. cv. Solara) were grown under ambient (360 mul l(-1)) or elevated (700 mul l(-1)) atmospheric CO2 concentration with or without the presence of the arbuscular mycorrhizal (AM) fungus Glomus caledonium. It was hypothesised that (1) the populations of free-living soil protozoa would increase as a response to elevated CO2, (2) the effect of elevated CO2 on protozoa would be moderated by the presence of mycorrhiza and (3) the presence of arbuscular mycorrhiza would affect soil protozoan numbers regardless of atmospheric CO2. After 3 weeks growth there was no difference in bacterial numbers (direct counts) in soil, but the number of free-living bacterial-feeding protozoa was significantly higher under elevated CO2 and was significantly reduced in the mycorrhizal treatments. These effects on protozoa disappeared after 5 and 9 weeks. Neither mycorrhiza nor CO2 concentration had any substantial effect on the microbial community structure as evaluated by phospholipid fatty acid analysis. The increased protozoan populations under elevated CO2 suggest increased bacterial production, whereas the lower populations in response to presence of mycorrhiza suggest a depressing effect on bacterial production by AM colonisation. (C) 2002 Elsevier Science Ltd. All rights reserved.