Weed flora and arthropod fauna were investigated in fodder beet plots located on six sites in Jutland, Denmark in June 2000. On each site three treatments were represented viz. traditional beets (T), glyphosate tolerant beets (RR) treated with full Roundup Ready dosage RR100), RR beets treated with half the dosage applied in RR100 (RR50). Roundup Ready were applied later than the traditional herbicides and for a period of time the weed density were higher in RR plots until some time after the second Roundup Ready was applied, then the RR plots became very clean. The arthropod fauna responded to the change in the amount of weed present in the RR plots. It is believed that the faunal response is mainly governed by three mechanisms: 1) the herbivorous fauna component experience more host plants and more flowers in weedy plots, 2) a number of non-herbivorous arthropods prefer the more gentle microclimate in weedy plots and the more complex vegetation structure (web building spiders), 3) predators are attracted by the high prey density in weedy plots including birds.