Students in three sections of a high school biology course were taught a unit on evolution and natural selection. Prior to instruction, students were pretested to determine their (a) reflective reasoning skill, (b) strength of religious commitment, (c) prior declarative knowledge of evolution and natural selection, and (d) beliefs in evolution or special creation and related religiously oriented beliefs. Following instruction the measures of declarative knowledge and beliefs were readministered. The study was designed to test (a) the hypothesis that the acquisition of domain-specific concepts and the modification of nonscientific beliefs largely depends upon reflective reasoning skill, not prior declarative knowledge; and (b) the hypothesis that strength of religious commitment and a belief in special creation hinder the acquisition of scientific beliefs. Although instruction produced no overall shift toward a belief in evolution, as predicted, reflective reasoning skill was significantly related to initial scientific beliefs, and reflective reasoning skill, but not prior declarative knowledge, was significantly related to gains in declarative knowledge. Reflective reasoning skill, however, was not significantly related to changes in beliefs. Also as predicted, strength of religious commitment was negatively correlated with initial belief in evolution and with a change in belief toward evolution. Interrelationships among the study's major variables, as well as educational implications, are discussed.