In the United States, dominant language ideology validates the use of Standardized English as the only appropriate linguistic practice (Lippi-Green, 1994; Flores & Rosa, 2015). In educational settings, dominant language ideology entails the beliefs in language standardization and monolingualism (Farr & Song, 2011). The present case study examines the beliefs and practices of one ESL teacher and investigates how dominant language ideology is reproduced and resisted in the adult ESL classroom. The study employs multiple sources and modes of data (a questionnaire, in-person observations, and a semi-structured interview) collected over multiple lessons. The study shows that the teacher generally resists dominant language ideology: she views students' code-switching as a resource, rather than a deficit and does not associate Standardized English with correctness. Accordingly, incorporating language variation in the ESL classroom is viewed as a necessary teaching practice. Nevertheless, the teacher reinforces the notion of the monolingual native speaker as the authority for linguistic expertise and, thus, aligns with the tenets of dominant language ideology. Ultimately, by offering an account of the beliefs and practices of one ESL teacher, the present study illuminates the ways in which teachers can resist dominant language ideology in the classroom and, thus, engage with critical language pedagogy.