Using an automatic volumetric spore trap for sampling air at 2 m. above ground in an open field at Rothamsted Experimental Station, Harpenden, the diurnal and seasonal periodicities of 15 spore types belonging to fungi pathogenic to plants were followed during June to September, 1953. Large numbers of smut spores and conidia of powdery mildews occurred very early in the season when spores of Botrytis and Periconia also reached their peak. Uredospores of rusts and Helminthosporium spores were in maximum numbers during the middle of the season and spores of Polythrincium and Epicoccum occurred in large numbers towards the end of the season. Alternaria and Stemphylium spores began to appear during the middle of the season and persisted till the end. Spores of Tilletia, probably liberated into the air during the threshing operations, were seen in appreciable numbers towards the end of the season. Spores of Ganoderma were present over the whole period, while those of Coniophora occurred over a very short time. It was found that in dry weather many showed typical diurnal periodicity. Ustilago, Helminthosporium, uredospores, Botrytis, Alternaria, Stemphylium and Periconia were most abundant in the afternoon and least in the early morning. Erysiphe, Polythrincium and Epicoccum also occurred during the daytime but reached their maxima before noon. Spores of Ganoderma, Coniophora and other coloured basidiospores were found to occur in great concentrations between midnight and dawn. The changes observed during this crop season were compared with the existing data and it was concluded that the changes described were, in general, representative of large rural inland agricultural areas in temperate regions.