Although the use of portfolios to assess student performance in K-12 science classrooms and to monitor the training of preservice science teachers is increasingly common, their implementation in undergraduate science courses is still limited. The work presented here represents one in-depth example of the integration of portfolio assessment into the standard assessment practices of a general education course for nonscience majors. The subject matter of the course was astrobiology, but the approach would work equally well for other science disciplines. The portfolio assignments were characterized by intensive writing, iteration on written assignments, engaging with popular and semitechnical articles on the subject matter, and expressing scientific principles involved in other contexts. The work described here will be of interest to other science instructors curious about ways in which to design portfolio assessments for use with nonscience majors to support general education requirements. (Contains 2 figures.)