Male and female Pekin ducks selected (F2 generation) for greater breast muscle thickness (MT) and an unselected control (C) were used in this study under mixed- or separated-sex rearing. Ducks in the separated-sex program had significantly higher body weights than the birds in the mixed-sex program. The ducks selected for greater breast muscle thickness exhibited superior (P < 0.05) body weight up to 42 d of age. Males from the MT line reached market weight at 6 wk, but the breast muscle thickness, measured by ultrasound scanning, improved significantly from 6 to 7 wk of age. Males had greater (P < 0.05) breast muscle thickness than females. At both ages and for both sexes, the MT line had greater (P < 0.05) breast muscle thickness than the C line. The MT ducks consumed more (P < 0.05) feed up to 6 wk, but no significant difference was found in cumulative feed consumption at 7 wk, and the lines did not differ (P > 0.05) in feed:gain ratio. Males had significantly higher cumulative feed consumption at 6 wk but had a more desirable (P < 0.05) feed:gain ratio than females at 7 wk. Compared with females from the C line, MT females had significantly higher carcass yield, higher body protein, and lower fat. The MT females had higher plasma glucose and lower uric acid than the C females, but no significant difference was found between the MT and C lines for plasma triglycerides and total cholesterol. The results of this study support the utility of ultrasound scanning in duck selection, the separated rearing of ducklings, and the possibility of reducing the slaughter age of lean males.