This study identified the types of sexist language that appear in ESL textbooks by Thai authors. The study analyzed the ESL textbooks by Thai authors sold at the Chulalongkorn University bookstore during spring 2007. It was a qualitative case analysis of fifteen ESL textbooks covering the beginning, intermediate, and advanced levels of ESL instruction. The study used feminist criticism to discover what gender roles are sanctioned as appropriate in ESL textbooks by Thai authors and if the language used supports or challenges patriarchy. The results of this study show that sexist language is present in the textbooks and that the textbooks contain content that promotes sexist assumptions concerning gender roles. As a whole, the language and examples used in ESL textbooks by Thai authors support patriarchy.