The author of this TIEE Research Paper assessed changes in students' understanding of experimental design during a semester-long, inquiry-based, plant ecology laboratory. Students learned about and designed experiments to address four hypotheses about invasive species. Students were given similar pre- and post- tests in both years. Student knowledge self assessment questions were added in 2005. Also in 2005, students analyzed an experimental design on an interim assessment. In 2004, 4 of the 8 questions showed a significant shift to more correct answers on the post-test. In 2005, 2 of the 10 questions showed a significant shift to more correct answers. In both 2004 and 2005, the percent correct answers per student on experimental design questions increased between the pre- and post-test. The majority of students correctly described 8 of 9 components of an experiment on the interim assessment. The results suggest that participation in the laboratory improved student understanding of experimental design.