Course-based Undergraduate Research Experiences (CURE’s) are unique opportunities for students to engage in a research experience that permits them to make discoveries and analyze data they collect (Dolan, 2017). One central aspect of CURE’s is that these experiences offer students the opportunity to learn about scientific practice and scaffold students into thinking more like scientists. While progress has been made in understanding what constitutes a CURE and the effect a CURE can have on student understanding of the nature of science, we currently do not have an observation protocol that assesses key aspects of student understanding of the nature of scientific inquiry (NOSI) within a CURE. We present a validation study on our NOSI protocol that examines how aspects of the NOSI can be characterized within student dialogue in CURE classrooms. We recognize that validation is an iterative process and demonstrate how the development of our protocol is in alignment with contemporary ideas about instrument validation (Kane, 2016). For this study, we collected audio and video data from 27 groups of 3-4 students at three time points over the course of a CURE in a second semester introductory course for majors that was held at a large research university in the Pacific Northwest. Our evidence of validation emerges from four of the major inference types within contemporary validation (domain description, evaluation, generalization, and explanation). For domain description, we will share evidence of themes that we chose to assess that are in alignment with recent literature on the NOSI. Second, evidence for the evaluation inference will describe how the structure and implementation of this protocol are representations of student understanding of the NOSI. Third, generalization will illustrate how scores on this protocol are consistent across raters. Finally, the explanation inference will be described with evidence in that student understanding of the NOSI is actually reflected in student scores on this protocol. This work has important implications for researchers and instructors as it is a potential tool for assessing student knowledge of the nature of scientific inquiry which is an important outcome for CURE’s.