The weed flora on 57 arable fields on 17 farms employing organic farming was recorded. With the ordination technique pCCA (partial Canonical Correspondence Analysis) the relative importance of some farming practices was evaluated (crop, preceding crop, ± undersown ley, ± weed harrowing, ± weed hoeing, ploughing regime, ± compost, ± animal husbandry, ± biodynamic agriculture). The variables that explained most of the variation in the weed flora in fields (along with typical weed species) were the crops ‘beans and peas’ (analysed together;Centaurea cyanus), ‘ley as preceding crop’(Cerastium fontanum), ‘weed hoeing’, ‘fallowing in the preceding year’ (Lamiumspp.) and ‘animal husbandry’(Sinapis arvensis).A number of species considered endangered, rare or decreasing in Sweden were recorded on these farms, suggesting that organic farming can contribute to maintaining biodiversity in an agricultural landscape. The weed flora was also compared with that of unsprayed control plots in herbicide trials. Ranking of weeds according to either frequency or a pCCA both showed clear differences, with organic fields having much more weeds. Species typical of organic farming wereCentaurea cyanusandVicia hirsuta.There was a tendency for weed species that dominate in conventional farming to be more nitrophilous than those species characteristic in organic farming.