Aster yellows, an insect-vectored disease caused by a mycoplasma-like organism (MLO), is a destructive vegetable plant disease in the upper midwestern United States. A breeding effort was initiated in 1982 to develop aster yellows resistance in carrot. An aster yellows synthetic (AYSYN) population composed of four standard open-pollinated cultivars and five inbred lines was assembled in 1982. Inbred lines were extracted from the AYSYN population using a variety of methods. Selection in naturally and artificially infested field sites was carried out from 1982 to 1989. Twenty-three carrot germplasm lines inbred for a minimum of five generations and three hybrids were developed from the AYSYN population during the selection process. Replicated field experiments were conducted in 1990, 1991, and 1993 to determine the relative aster yellows resistance of these 26 selected lines compared with six commonly grown carrot cultivars. Susceptibility was based on the presence of disease symptoms on carrot shoots. Data were collected as percent aster yellows infection based on the presence or absence of crown shoots on each plant. Averaged over years, significant differences were detected for percent aster yellows infection among carrot genotypes. Breeding lines selected for resistance ranged from 2.5% to 35.3% infected plants per plot averaged over years, while standard cultivars ranged from 12% to 42% infected plants per plot. Significant reductions in aster yellows infection were observed in many selected lines compared to standard cultivars. Results demonstrate the effectiveness of inbreeding and selection for aster yellows resistance. This breeding effort represents the first report of aster yellows-resistant carrot germplasm.