Seasonal variations in the capacity of propagules of vesicular-arbuscular mycorrhizal (VAM) and ectomycorrhizal (ECM) fungi to form associations in natural habitat soils were investigated by bioassays. These bioassays involved bait plants which? were grown in intact cores of forest soil to assess mycorrhiza formation by hyphal networks and other propagules of YAM or ECM fungi. Soil cores were collected from the same positions along a transect at different times of the year from a diverse sclerophyllous forest community dominated by Eucalyptus marginato (jarrah) and E. calophylla (marri) trees in the mediterranean (winter rainfall) climatic zone of Western Australia. Seasonal fluctuations in mycorrhiza formation by propagules of VAM or ECM fungi, or colonization by particular VAM endophytes (identified by their morphology within roots), were not substantial. Thus hyphal networks and other mycorrhizal fungus propagules in this soil maintained u similar capacity to colonize roots throughout the rear, even during extended periods of summer drought. It appeared That low light levels in the glasshouse could reduce the capacity of bait plants to form mycorrhizas during the winter. Substantial spatial variability in mycorrhizal fungus inoculum levels between the transect locations where cores were collected persisted through time, but variations in the relative cover of host or non–host plants at these locations were not sufficient to explain these patterns.