A structural equation model of expertise in college physics was tested on a sample of 374 college students in two different level physics courses. The variables that characterize expertise in physics including strategy use, pictorial representation, categorization skills, and motivation were examined for their influence on achievement in physics. Gender was included in the model to determine its impact on physics achievement. Results suggested a similar model across both level courses: Student motivation had a significant influence on students strategy use and categorization. Categorization, in turn, influenced student achievement directly, and indirectly, through strategy use. Strategy use also had a significant influence on achievement. Pictorial representation basically played no role in the model. Gender played an important role in the model, particularly for the more advanced level course for physics and engineering majors. Implications of these findings for physics instruction are discussed.