Differences in population growth patterns of two nematode species, Aphelenchoides bicaudatus and Aphelenchus avenae, were observed on food sources of agar plate cultures of six fungi, Cladosporium cladosporioides, C. herbarum, Fusarium chlamydosporum, Penicillium simplicissimum and P.restrictum strains I and II. The population, measured as biomass or individuals cm-2, of A. bicaudatus was bigger than that of A. avenae after 20-23 days at the end of the experiment with most of the fungi. However, the population growth of A. avenae on P.restrictum (I) was faster after the first 7 days than that of A. bicaudatus. On cultures of F. chlamydosporum and C. herbarum, the population growth of both nematodes was greater than on the other fungi. C. cladosporioides did not support the reproduction of either nematode.