He introduced the Billionaires Club of Duckburg, where Scrooge McDuck is a member, in the story "The Christmas Cha Cha" (1959), written by Gregory and illustrated by Carl Barks. After having done Scrooge McDuck universe comics only in his early career, he started doing Aristocats stories in the 1970s, when he also penciled most issues of Daisy and Donald, co-starring Daisy Duck.