Implementation of a guided-inquiry lab in introductory biology classes, along with scaffolded instruction, improved students' understanding of the scientific method, their ability to design an experiment, and their identification of experimental variables. Pre- and postassessments from experimental versus control sections over three semesters showed that most students improved in their understanding of the scientific method and experimental design skills. Students exhibited improvement in their ability to create hypotheses and correctly identify controls and dependent variables. However, students in both groups struggled with the identification of independent variable and controlled variables.