The longissimus thoracis (LT) from USDA Select beef carcasses was used to test the effect of aging period (0, 7, 14, 21, 28, or 35d) on bloom development (n=10/aging period). After aging, two 2.54-cm-thick, non-adjacent steaks were cut and instrumental color of the LT was measured at 10-min intervals for 2h. All instrumental color parameters increased (P<0.05) over 40% during the first 10min; however, asymptotic points indicated that color plateaued sometime after 120min. Although aging did not (P⩾0.05) affect the values where color development plateaued, rate of oxymyoglobin formation tended to be the slowest (P=0.06) in LT steaks aged 7d compared to steaks aged 0, 14, 28 and 35d. Results indicated that as much as 90% of the total increase (P<0.05) in instrumental color and oxymyoglobin percentages was achieved during the first 60min after cutting, but aging period had no impact on bloom development.