Within a sociocultural frame, teacher knowledge finds its origin in the entirety of teachers' lived experiences, not just those experiences within teacher preparation. Teachers' biographies, including their experiences as language learners, shape their knowledge base for teaching English to speakers of other languages (ESOL). This study interrogates one element of that knowledge base: teachers' linguistic knowledge for teaching. Cases studies of two early career ESOL teachers with similar language learner biographies, that of first language (L1) Center English speakers with limited second language (L2) learning experience, provided insight into the ways participants' language biographies informed their linguistic knowledge for teaching. Findings indicated that participants' L1 knowledge of English did not provide them with the linguistic knowledge they needed for ESOL teaching. Implications for ESOL teacher education include better attuning teacher preparation programs to teacher candidates' biographies.