Virology is an integral part of introductory biology courses. Research and experience indicate that, although the topic is a difficult one for many students, the effectiveness of instruction is enhanced when assignments actively engage the students in the generation of scientific explanations. These authors have found that the methods and applications of virology can be effectively demonstrated using a hands-on investigative laboratory exercise. Students act as virus hunters, monitoring the presence of coliphage in various field-collected water samples by direct plaque assay. They are responsible for data collection, analysis, and interpretations of results. The purpose of this activity is to promote both content mastery and critical thinking through self-discovery. This laboratory exercise engages students to: (1) improve their scientific reasoning and communication skills; (2) determine and modify experimental protocols; (3) organize and analyze quantitative data; and (4) draw conclusions and communicate their reasoning. (Contains 2 figures and 1 table.)