MILA-S is an interactive open learning environment for sci-entific modeling (Joyner, Goel, & Papin, 2014). It enablesstudents to build conceptual models of ecological phenom-ena, evaluate them through simulation, and revise the modelsas needed. MILA-S automatically spawns simulations fromthe conceptual models, making modeling easier for the stu-dent. Earlier work had described the use of MILA-S in middleschool. In this paper, we report an experiment on the use ofMILA-S in two college-level classes. In one class, we foundthat almost half of the students showed improved understand-ing of scientific modeling; in the other class, about two thirdsof the students showed enhanced understanding.