Scymnus marginicollis Mannerheim was reared on green peach aphids, Myzus persicae (Sulzer). At 20°–25 °C and a 16-h photoperiod, durations of egg, larval, and pupal stages were about 5, 10, and 7 days, respectively. Adults survived more than 80 days. The ingestion of wax-producing prey was not required for the beetle larvae to produce their woolly wax covering. Larvae fed exclusively by sucking fluids from appendages of the aphid prey. The coccinellid consumed about 16 adult aphids during the larval stage. Adults fed on fluids and tissues of M. persicae. Female adults consumed about five aphids per day, nearly twice as many as did males. The beetle populations increased in July and reached peak abundance in August. The seasonal trend of S. marginicollis suggests one generation per year.