Poor winter survival is the most important factor limiting the use of perennial ryegrass (Lolium perenne L.) cultivars as persistent turfgrasses in cold, temperate regions of the United States and as a persistent seed production crop in northern Minnesota. A comprehensive survey of available germplasm for winterhardiness has not been published to date. Three-hundred wild and semi-wild accessions of perennial ryegrass obtained through USDA-GRIN and collections at Rutgers University were planted with 9 check cultivars and populations of known winterhardiness as spaced plants in a randomized, blocks within replications design in August 2004. Ten incomplete blocks of 30 accessions and 9 checks each were nested within each of three replications at each of two environments: the St. Paul Agricultural Experiment Station, St. Paul, MN, and the Sand Plains Agricultural Experiment Station, Becker, MN. Five spaced plants represented a single entry for each accession or check, although ratings were performed on each plant as a separate unit to monitor the effect of variation within accessions. In May 2005, plants were rated for tiller survival (1 to 9, with 1 representing plant death and 9 representing almost complete tiller survival). A harsh winter with little to no snow cover and extreme fluctuation of temperatures led to complete death of all perennial ryegrass genotypes at the Becker, MN, environment and survival of at least one plant of 171 accessions at the St. Paul, MN, environment. Survival ranged from regrowth of a single tiller on a plant to almost complete tiller survival. These data will allow breeders to make use of existing germplasm resources to improve the winterhardiness of perennial ryegrass through traditional plant breeding.