Results of several studies on the relationship between science students' perceptions of their classroom environments and their cognitive and affective achievements indicate strong association between the perceptions and the achievements. While this underscores the need for more effective relations in the classrooms, the issue of the effectiveness of some contemporary instructional strategies for teaching science in different classroom environments has not been addressed. This study, therefore, sought to find out if the constructivist instructional strategy can enhance cognitive and affective achievements of students in non-conducive environments. A total of 100 (57 boys, 43 girls) junior secondary two (grade 8) students participated in this experiment. Two instruments-the Cognitive Achievement Test and the Affective Achievement Test-were used in pre and post-test administration to measure the treatment effect on cognitive and affective achievements, respectively. Findings do not support the stand that the constructivist instructional strategy is more effective than the traditional (expository) teaching strategy for improving cognitive achievement. But with respect to affective achievement, the evidence supports the use of the constructivist strategy for instruction in non-conducive classroom environment. Implications of the study are discussed and recommendations given.