Evaluation of Puccinia crupinae, the causal agent of a rust disease on common crupina (Crupina vulgaris), for biological control is described. Susceptibility of accessions of common crupina that represent both varieties of the target from the five populations in the United States indicate that the disease has potential to control common crupina, but differences were noted between accessions on the basis of pustule count, yield (i.e., number and weight of achenes per plant), and shoot dry weight data after multiple inoculations. One accession from Modoc, CA, was not affected in greenhouse tests and would likely not be affected in the field if a permit to release P. crupinae were granted. None of the nontarget species of 26 taxa from the tribes Cardueae and Cichoriae were symptomatic, so the pathogen is likely safe to use in North America.