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Timothy (Phleum pratense L.) is one of the most winter-hardy, cool-season perennial grasses used for forage production. However, its lack of tolerance to water stress requires supplemental irrigation for successful production in more arid areas of the world. A set of 112 timothy accessions and 6 check cultivars were evaluated for dry matter yield, in vitro true digestibility, neutral detergent fiber, crude protein, and water soluble carbohydrates across two production years at Lewiston, UT, and Tetonia, ID. Supplemental irrigation was not supplied at Tetonia. For each of the traits except water-soluble carbohydrates, the variance due to the main effect of accessions differed from zero. There was also accession x environment interaction for each of the traits. Within-environment broad-sense heritability estimates differed from zero and ranged from 0.45 +/- 0.16 to 0.80 +/- 0.40. Thus, genetic determinants exerted substantial influence on the expression of the traits in each environment. Most of the commercial cultivars exhibited no benefit over the accessions for these traits, and there was a relationship between the accessions’ geographic origin and their potential for timothy improvement. Asian and Northwest and Northeast European accessions possessed the greatest potential for increasing timothy nutritive value under western North American conditions. Yet there was little evidence that any accessions would provide a yield improvement.