The present study included repeated ultrasound-predicted percentage of intramuscular fat (UPFAT) measures from 675 Angus bulls and heifers collected over a three-year period between 1998 and 2000. Each year cattle were scanned four to six times for UPFAT and other ultrasound traits, starting at a minimum age of 28 weeks. The objective of the current study was to estimate variance components, heritability, and repeatability of UPFAT in young bulls and heifers. Heritability of UPFAT ranged from 0.32 at 28 weeks of age to a maximum of 0.53 at 63 weeks. Repeatability of UPFAT increased from a minimum of 0.60 at ages 28 to 39 weeks to a maximum of 0.80 at ages 61 to 63 weeks. Heritability and repeatability of yearling UPFAT was 0.50 and 0.71, respectively. The present results showed an optimum heritability and repeatability of UPFAT measures around 52 weeks and through at least 63 weeks of age. This suggested that differences in UPFAT measures during this period also are good measures of differences in marbling genetic potential of Angus cattle.