International assessments have revealed that students in numerous nations are lacking scientific reasoning skills [1,2]. Science teachers who support students’ scientific skill development through the use of authentic practices provide students with tools needed for success in future science courses [1,2,3,4,5]. Teachers training focused on pedagogy that supports student scientific reasoning development is particularly important as some studies have suggested that preservice teachers have a tendency to display a lack of scientific reasoning skills as well . Additionally, few studies exist that assess teachers’ scientific reasoning skills, including the effectiveness of professional development to strengthen teacher scientific reasoning abilities over time. To help fill this gap, this study examines the effects of a Modeling Instruction in biology workshop on teachers’ scientific reasoning skills. In addition to teacher interviews, focus groups, and writing samples, Lawson’s Classroom Test of Scientific Reasoning (LCTSR)  data were collect from teachers before and after the workshop. The results suggest that the three-week Modeling Instruction in biology workshop contributed to gains in inservice teachers’ scientific reasoning, and thus providing evidence that the teachers in this study are more prepared to help develop similar skills within their own students as they engaging in the Modeling Instruction curriculum.