This study examined two hypotheses related to same-gender grouping of eighth grade science classes in a public middle school setting. The hypotheses were (a) male and female students enrolled in same-gender science classes demonstrate more positive science academic achievement than their peers enrolled in mixed-gender classes, and (b) same-gender grouping of students has a positive effect on classroom climate. Participants were randomly assigned, and instruction did not vary for the same-gender and mixed-gender classes. The first experimental group was a class of all-male students (n = 20) taught by a male science teacher. The comparison group consisted of male students (n = 42) in coeducational classes taught by the same male teacher. The second experimental group was a class of all-female students (n = 23) taught by a female science teacher. This comparison group consisted of female students (n = 61) in coeducational classes taught by the same female teacher. Results of this study led to the conclusions that same-gender grouping did not produce significant differences in student science academic achievement and same-gender classes did not create a more positive classroom climate. For this middle school, policy recommendations include limiting same-gender grouping until further research indicates that this structure is effective in countering gender-stereotypes and improving achievement and attitudes toward science. (Contains 2 tables.)