Undergraduate biology lab courses have long been criticized for engaging students in "cookbook" experiences in which students follow a given protocol to collect data that help answer a predetermined question. Recent reform documents in biology education have suggested that students should engage in lab courses that provide more authentic research experiences in which students are responsible for the development of research questions, the collection of data, and its analysis. This paper presents the evaluation of a research-based introductory biology lab course focused on an ecological system. Pre- and postcourse surveys and performance assessments were administered to nonvolunteer students to measure the course's impact on cognitive and affective constructs. Results indicate that participation in the authentic lab course improved students' experimental design and data interpretation abilities at a statistically significant level. Furthermore, students' confidence in their ability to execute authentic lab tasks as well as their perception of the lab's purpose changed over time, both in ways that reflect more authentic engagement in scientific research. (Contains 4 tables and 4 figures.)