Plant species richness and cover of 698 samples of weed flora, recorded in standard plots in the Czech Republic from 1955 to 2000, were related to altitudinal floristic regions, soil types, cultivated crops, climate, altitude and year of the record. Stepwise backward elimination of explanatory variables was used to analyse the data, taking into account their interactive nature, until the general linear model contained only significant terms. Net effects of particular variables on weed species number and cover, independent of covariance with other variables, were determined.Weed species number and cover were significantly affected by altitudinal floristic region and its interaction with the year of sampling. Both weed species number and cover decreased over time, more so in the moderate-to-cold than in the warm altitudinal floristic region, due to the increase in agricultural intensification being more profound at higher than lower altitudes. There was no direct effect of soil type on weed species number, whereas the decrease of weed cover with increasing crop cover was more pronounced on nutrient-poor than nutrientrich soils. Maize fields contained the lowest number of weed species, while root crops and fodder plants were most species rich. Within the group of other cereals than maize, spring barley and oats harboured more weed species than winter wheat and, in particular, than rye. The differences in weed flora were largely attributable to management and partly related to crop-specific agricultural practices as well as general changes in the management of arable fields over the last decades.