The federal Conservation Reserve Program (CRP) funded the conversion of eroding cropland to grass or grass-legume cover that was not to be tilled, hayed, or grazed for 10 yr. It was not known what the species composition of CRP fields would be after years of minimal disturbance. Our objective was to document the presence and percentage groundcover of weeds in 151 CRP fields located in 10 Minnesota counties; including 108 Conservation Practice (CP)-1 (cool-season legumes and grasses) fields, 17 CP-2 (native grasses) fields, and 26 CP-10 (existing vegetation) fields. Groundcover of each species present and of bare ground was scored in six 106-sq-ft sample plots per field. The most prevalent species were the primary noxious weed Canada thistle [Cirsium arvense (L.) Scop.], the secondary noxious weed quackgrass [Elytrigia repens (L.) Desv. ex. Nevski], and the non- noxious weeds dandelion (Taraxacum officinale Weber.) and goldenrod (Solidago spp.). Weed percentage groundcover was higher in CP-10 fields than in CP-1 or CP-2 fields, probably because many CP-10 stands were already thinning at the start of the CRP contract. Volunteer legumes and grasses were common in CP-10 fields. In CP-1 fields, legume and grass percentage groundcover usually was correlated negatively with weed percentage groundcover. Weed percentage groundcover and species richness were correlated positively. Gopher mounding was correlated positively with the amount of bare ground and with the percentage groundcover of annual and biennial weed species. Primary, secondary, and non-noxious weeds were each found in nearly 90% of the fields studied. Widespread presence of noxious weeds on CRP fields is a cause for concern. Weed control issues should be addressed in planning a new CRP.