From January 9, 2000, to May 14, 2006, Sullivan appeared in the
Fox sitcom
Malcolm in the Middle as Malcolm's younger brother Dewey. In 2005, along with
Malcolm co-star
Jane Kaczmarek, he wrote the afterword to the children's book
Together, which is about farming and was inspired by the
nonprofit organization Heifer International. Sullivan had a starring role in the 2004 movie
Christmas with the Kranks and has played a variety of characters in film and on television, including the title role in the independent film
Mo (2007), as well as voicing Sheldon the Seahorse in both the animated film
Finding Nemo and its
video game tie-in. Sullivan's last acting credit was an appearance in the 2010 film
Twelve when he was 18 years old. He has since kept a low profile, and has been absent from several
Malcolm in the Middle cast reunions. Sullivan also did not return for the show's revival, ''
Malcolm in the Middle: Life's Still Unfair'', with Caleb Ellsworth-Clark taking over the role of Dewey. Kaczmarek said in 2025 that Sullivan was studying
Victorian literature. He turned down a chance at participating in the revival because it would take him away from his studies. ==Filmography==