In 1998, 1,260 soil samples were collected from 63 of 99 Iowa counties to characterize the weed seedbanks in fields under the conservation reserve program (CRP) and adjacent fields under continuous cultivation. Five annual grass and 13 broadleaf weed species were identified in both the CRP and adjacent cultivated fields. Seedbank differences between CRP and adjacent cultivated fields were evident only for foxtails, common lambsquarters, pigweeds, and sweetclover, with the average of 3,288, 10,681, 38, and 1,709 seeds/m2, respectively; the corresponding seed population in adjacent CRP fields was 59, 57, 1,924, and 74%, respectively. However, weed species diversity was not significantly different between fields in CRP and continuous cultivation. Only CRP fields in the northwest Iowa crop-reporting district had a higher foxtail species seed population (4,915 seeds/m2) than the adjacent cultivated fields (1,782 seeds/m2). Land under CRP in northern (N), eastern, and southern (S) districts had 58% (4,158 seeds/m2), 6% (312 seeds/m2), and 18% (594 seeds/m2) of the continuously cultivated foxtail species seedbank. Common lambsquarters seed populations were 4,128 and 3,801 seeds/m2 in the cultivated fields of the N and central (C) districts, compared with 772 and 252 seeds/m2 in adjacent CRP fields, respectively. Pigweed species seeds were more numerous in the cultivated fields than in adjacent CRP fields in the northeast, C, and S Iowa districts. Sweetclover seed population was consistently higher in CRP land because it was included as part of the CRP covers seeding. Overall, broadleaf weed seeds comprised 90% of the seedbanks in both CRP and adjacent cultivated land. A competitive cover crop canopy in CRP probably reduced weed seedbanks by suppression of weeds and seed production. Also, annual seed production, differences in weed biology, and differential herbicide performance in cultivated fields may have contributed to higher seed populations.