The determinants of intentions to enroll in a high school science course were investigated, using the theory of reasoned action, among earth science students enrolled in a middle school located in a middle-income, suburban community in central Texas. The sample consisted of 5 of 14 eighth-grade earth science classes, randomly selected for this study. Classes contained Caucasian and minority students, male and female, of differing science abilities who were grouped according to general academic abilities—basic, average, and gifted and talented. The prediction of behavioral intention of sample participants was tested using four external variables, attitude, and subjective norm (Direct-Full Effects Model), attitude and subjective norm alone (Direct-Reduced Effects Model), and disaggregated data on attitude and subjective norm (Indirect Effects Model). Results of the study revealed attitude and subjective norm to be the sole predictors of behavioral intention for the aggregated data, but to be differentially effective for groups formed on the basis of sex, ethnicity, general ability, and science ability. Evidence is presented to show that the relative contributions of attitude and subjective norm to the prediction of behavioral intention varies among students depending upon their sex, ethnicity, general ability, and science ability. Results of the study are discussed in terms of increasing the enrollment of all students in elective science courses.