Understanding students' thinking about scientific theories is fundamental to the development of effective instructional strategies designed to foster scientific literacy. We conducted a study to determine student acceptance of important biological theories and to explore the relationships between their acceptance of scientific theories and their understanding of the nature of science. Overall, students entering a nonmajors biology course (1) were undecided as to whether evolutionary theory is scientifically valid and explanatory and (2) accepted evolutionary theory at a lower rate than other theories in the field. Student understanding of the nature of science was not significantly related to their acceptance of any of the scientific theories in the study. (Contains 3 tables and 2 figures.)