The gluteus medius (GM) from USDA Select beef carcasses was used to test the effect of aging period on bloom development. Top sirloin butts (IMPS #184) were randomly allocated to 0, 7, 14, 21, 28, and 35d vacuum-aging at 2°C (n=10/aging period). Each week, aged top sirloin butts were faced before two 2.5-cm-thick, non-adjacent steaks were cut and instrumental color (L(∗), a(∗), and b(∗)) of the GM was measured at 10-min intervals for 2h after cutting. Steaks aged for 7 and 14d were a more vivid (greater chroma values; P<0.05), redder (greater a(∗) values; P<0.05), and more yellow (greater b(∗) values; P<0.05) color than steaks from the other aging periods. Change in total color (ΔE) was greater (P<0.05) for steaks from top sirloin butts aged 7, 14, and 21d than steaks from top butts aged 28 and 35d, whereas oxymyoglobin percentages for steaks from top butts aged 7 and 14 days were greater (P<0.05) than those from top sirloin butts aged 28 and 35d. As much as 90% of the total increase (P<0.05) in a(∗), b(∗), and chroma values, as well as hue angles and oxymyoglobin percentages, was achieved during the first 60min after cutting.