We studied 630 acres of roadside along 23 miles of Interstate 94 in Stutsman County, North Dakota, to assess wildlife values of highway rights-of-way. We found 422 duck nests that had an overall success of 57 percent in 1968, 1969, and 1970. Mammalian predators were responsible for 85 percent of the destroyed nests. To test the effect of mowing on duck nest initiation and success, alternate 1-mile blocks of the study area were not mowed in the fall of 1968. In 1969 and 1970, significantly more ducks chose unmowed vegetation in preference to mowed vegetation for nest sites. Mallards (Anas platyrhynchos), pintails (A. acuta), and gadwalls (A. strepera) were especially responsive to unmowed vegetation. Success of duck nests in unmowed vegetation was 62 percent compared with 51 percent in mowed vegetation. Sixteen percent of the nests were unhatched by July 5, the beginning mowing date previously recommended by the North Dakota Highway Department. Wildlife killed by traffic did not increase when half the mile blocks were unmowed, and no significant difference was observed in buildup of snow between mowed and unmowed blocks in the winter of 1968-69. Of 182 motorists interviewed in the study area, 82 percent had not noticed the unmowed rights-of-way. We strongly recommend no mowing of ditch bottoms or back slopes, minimal mowing of inslopes, and no mowing before July 20 to enhance waterfowl nesting and to reduce maintenance costs of highway rights-of-way in duck-producing regions.