International assessments have revealed that students in numerous nations lack scientific reasoning skills. 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. Teachers training focused on pedagogy that supports student scientific reasoning development is particularly important as some studies have also suggested that pre-service teachers have a tendency to display a lack of scientific reasoning skills. 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 a biology workshop on teachers' scientific reasoning skills. In addition to teacher interviews, focus groups, and writing samples, data from Lawson's Classroom Test of Scientific Reasoning (LCTSR) were collected from teachers before and after the workshop. The results suggest that the three-week Modeling Instruction in the biology workshop contributed to gains in in-service teachers' scientific reasoning, and thus provides evidence that the teachers in this study are more prepared to help develop similar skills with their own students as they engage in the Modeling Instruction curriculum.