Sixteen selections of Apocynum cannabinum (Indian Hemp) and nine of Asclepias syriaca (Common Milkweed) from midwestern USA were exposed to 40 or 80 ppb ozone under controlled conditions within greenhouse continuously stirred tank reactor (CSTR) chambers to evaluate their relative ozone sensitivity. The incidence and severity of ozone-induced symptoms on both species were directly related to ozone concentration and duration of exposure. The most common foliar symptom was classic, dark, adaxial stipple, similar to symptoms ascribed to ambient ozone in the field. Indian Hemp was more sensitive to ozone than Common Milkweed. Both species exhibited considerable intraspecific variation in ozone sensitivity. Variability in the data was too great to assign definitive ozone-sensitivity ratings within geographic regions from which seed was selected. However, two locations were identified as possible collection sites for ozone-sensitive selections of both species: Wabaunsee County, KS and Plattsmouth, NE for Indian Hemp; and Cloud County, KS and Swan Creek Lake Wildlife Area, NE for Common Milkweed. Plants derived from seed from these locations may serve as ozone-sensitive bioindicators.