In early 1998, the Yawn was one of the many test subjects bred specifically as experimental B.O.W.s. The specimen escaped, however, and managed to avoid recapture by the scientists in the Arklay Laboratory. It was eventually infected with the t-Virus following the leak in May 1998, and it promptly made the mansion its home, preying on any humans that happened to come across it.
The Yawn encountered S.T.A.R.S. Bravo team members, Richard Aiken, and Rebecca Chambers, as they explored the mansion, hunting them through individual rooms until it cornered them. After Richard managed to save Rebecca from harm, the Yawn attacked and bit into Richard before escaping back into the Mansion's framework. Bleeding and injected with a large dose of venom, Richard and Rebecca hid until they were discovered by S.T.A.R.S. Alpha Team members Chris Redfield and Jill Valentine.
The Yawn was later fought in the attic but escaped through a fireplace before it could be killed. The Yawn would later attack one of the S.T.A.R.S. Alpha team members in the library, where it was finally killed.