The purpose of the study is to investigate the types of corrective feedback used by a Thai teacher with adult learners of different proficiency levels in speaking classes in order to facilitate students' language development, as it is an aspect language teachers need to be aware of when teaching speaking. Three different levels of speaking classes were observed, and the audiorecordings were transcribed and analyzed to find out the corrective feedback strategies used based on the model by Lyster and Ranta (1997). The findings reveal that recasts were the corrective feedback strategy most frequently employed by the teacher across all three proficiency levels because it does not interrupt the flow of communication. As recasts and explicit feedback technique are the two most used strategies in correcting students' errors, it can be said that the teacher in the study usually provided input corrective feedback rather than elicit the correct form from the students. The study suggests that language teachers should be aware of making decisions regarding the types of corrective feedback that would be more appropriate to each student's level in the Thai context of English learning, particularly in an adult learner speaking classroom.