This study explores methods for detecting gender-based differential item functioning (DIF) on the 12th grade multiple-choice and constructed-response science tests administered as part of the National Education Longitudinal Study of 1988 (NELS:88). Several combinations of conditioning variables were explored for DIF detection on both tests, and results were supplemented with evidence from interviews of 25 students who completed the test items. On both tests, DIF in favor of males was exhibited primarily on items that involved visualization and that called upon knowledge and experiences acquired outside of school. The findings reveal that neither content nor format alone explained the patterns of male and female performance, and that an investigation of response processes may provide valuable additional information about the nature of gender differences in science achievement. (Contains 7 tables, 4 figures, and 56 references.) (Author/SLD)