When the Mario Bros. get to the dead-end of Yoob's Belly, they encounter Sunnycide who has captured the Yoshis and Baby Bowser inside eggs but Baby Bowser frees himself. The Mario Bros. and Sunnycide fight. Sunnycide attacks the Mario Bros. by throwing eggs at them which they can jump over or on top of, or getting health from them by sucking them into its mouth; however, Baby Mario and Baby Luigi can whack its sensitive part with a hammer. All attacks are useless against it until the Mario Bros. free all the Yoshis from the eggs to roll a boulder on it which causes it to fall over and then the Mario Bros. can jump on top of its sensitive part inside its mouth. When Sunnycide is knocked down, it can do two attacks; it can pull a chain to create a hole for Mario or Luigi to fall in, but they can avoid falling by jumping at the right moment. It can also pull a switch to dim the lights, summoning a Boo to attack the Mario Bros., so the Boo can be avoided by having the Baby Mario Bros. whack it before it touches them.
When the Mario Bros. kill Sunnycide, an entrance opens which makes everyone leave Yoob's belly causing it to presumably die.
- Sunnycide's name is a play on the term "sunny-side up", a way that people fry eggs.