Science instructors assume that they have created their laboratory curriculum in such a way as to reflect an ideal science instructional laboratory, but students may not recognize this. A recent study used the Student Perceptions of the College Instructional Laboratory Survey (SPCILS) to examine students' perceptions of their science instructional laboratory as intended by the instructors. What this study did not indicate, though, is whether students more interested in science perceive the science instructional lab as intended by the instructors. Because an appropriate instrument is not available, researchers have not been able to assess students' casual interest in science. Interest, however, is considered one of the most effective motivational forces of learning. Hence, we developed the Casual Interest in Science Survey (CISS). Employing the CISS, this study used a quantitative design to examine undergraduate students' perceptions of their science instructional laboratory with respect to their casual interest in science. Interest and perception data were collected at a Midwestern University in the USA. The findings suggest that undergraduate students are interested in science. Major status influenced students' perceptions of their science instructional laboratory with respect to their casual interest in science. However, gender does not influence students' perceptions of their science instructional labs with respect to casual interest in science. This study provides baseline data for future qualitative studies about how major and gender might be influencing students' perceptions of their science instructional laboratory with respect to their casual interest in science.