The Australian, aiming to become the first swimmer to win the same event at three successive world championships, clocked three minutes 47.44 seconds in a comfortable swim to win the final heat from promising young Russian Yuri Prilukov (3:48.49).
Thorpe became the youngest men's world champion ever when he won the 400m freestyle in Perth in 1998 at the age of 15. He retained the title in Fukuoka, Japan, in 2001 when he won six gold medals, a record at a single world championships.
Team mate Grant Hackett, silver medallist behind Thorpe in Perth and Fukuoka, won the preceding heat in the second-fastest overall time of 3:48.35
Rosolino, who was silver medallist behind Thorpe at the 2000 Sydney Olympics, clocked 3:49.59, finishing just ahead of Britain's Graeme Smith (3:49.92).
Both qualified for the evening's final. But fellow Italian Emiliano Brembilla, bronze medallist in 2001, failed to make it, touching a distant sixth in his heat in 3:53.43.
American Jenny Thompson, the 1998 world champion, set a championship record 58.14 seconds to lead the semi-final qualifiers in the women's 100m butterfly.