The purpose of the study was to measure the effects of higher level, inquiry-based science curricula on students at primary level in Title I schools. Approximately 3,300 K-3 students from six schools were assigned to experimental or control classes (N = 115 total) on a random basis according to class. Experimental students were exposed to concept-based science curriculum that emphasized "deep learning" though concept mastery and investigation, whereas control classes learned science from traditional school-based curricula. Two ability measures, the Bracken Basic Concept Scale-Revised (BBCS-R, Bracken 1998) and the Naglieri Nonverbal Intelligence Test (NNAT, Naglieri 1991), were used for baseline information. Additionally, a standardized measure of student achievement in science (the MAT-8 science subtest), a standardized measure of critical thinking, and a measure for observing teachers' classroom behaviors were used to assess learning outcomes. Results indicated that all ability groups of students benefited from the science inquiry-based approach to learning that emphasized science concepts, and that there was a positive achievement effect for low socio-economic young children who were exposed to such a curriculum.