Two experiments were conducted simultaneously to evaluate the effects of supplemental ground flaxseed on site and extent of digestion and growth performance in beef cattle grazing summer native range. Six Angus heifers (initial BW 367 +/- 8.0 kg) fitted with ruminal and duodenal cannulas were used in Exp. 1, and 18 Angus cross steers (initial BW 368 +/- 4.6 kg) were used in Exp. 2. Cattle from both experiments were allotted to 1 of 3 individually fed treatments: grazing only (CON), grazing plus a cracked corn-soybean meal supplement fed at 0.32% of BW once daily (CRN), or grazing plus ground flaxseed fed at 0.18% of BW once daily (FLX). In Exp. 1, supplement did not affect (P = 0.24) masticate in vitro OM digestibility; however, between supplemented treatments, cattle fed FLX tended (P = 0.10) to select a lesser quality masticate than corn-fed cattle. Forage OM intake was not affected (P = 0.17) by supplementation, nor was there a difference (P = 0.51) between CRN and FLX. A quadratic (P = 0.001) response was observed for forage OM intake as the grazing season advanced. Duodenal and fecal OM flows were not different (P >or= 0.42) across treatments. Therefore, true ruminal and total tract OM digestibilities did not differ (P >or= 0.40) between CON and supplement treatments, and total tract digestibility was greater (P = 0.04) for CRN than FLX. Total duodenal N flow did not differ (P = 0.20) across treatments, but responded quadratically (P = 0.03) with advancing season. True ruminal N digestibility was not affected by supplementation (P >or= 0.20). Likewise, ruminal NDF digestibility did not differ (P = 0.29) with supplementation, and CRN was not different (P = 0.27) from FLX. In Exp. 2, there was a treatment x period interaction for forage intake (P < 0.001), ADG (P = 0.001), and feed efficiency (P < 0.001). Supplement did not change (P = 0.34) forage intake compared with CON, but it was greater for CRN than FLX (P < 0.001). Nevertheless, ADG was greater for supplemented steers (P < 0.001) compared with CON, but did not differ (P = 0.41) between CRN and FLX. Feed efficiency was improved (P < 0.001) for supplemented steers and was greater (P = 0.01) for FLX than CRN. Although ground flaxseed reduced digestibility compared with a corn-soybean supplement, this reduction in diet digestibility did not negatively affect the growth performance of grazing steers.