A study investigated the effects of a prior knowledge activation strategy on students' ability to successfully engage in the ongoing comprehension of extended text. Data were elicited from 79 ninth graders who were enrolled in a required social studies class in an urban high school in the Southeast. Equal numbers of high and low readers were assigned to each of three treatment groups: prior knowledge activation (PKA) group; main idea (MI) group; and control group. All subjects participated in eight 45-minute instructional sessions followed by four 45-minute testing sessions (three immediate testing sessions and one delayed testing session 4 weeks later). Naturalistic reading passages were selected from the social studies textbook that the students were using, and instructors were mixed across treatments to control for an instructor effect. Results revealed that explicit instruction in how to spontaneously activate prior knowledge during reading had a positive affect on students' ability to answer application level questions. Results also revealed, however, that PKA strategy did not have a significant effect on literal comprehension, and that the MI group performed better on the literal measure, since the treatment focused on encouraging students to target main ideas and supporting details rather than to elaborate from the text. Further research into a strategy combining the tasks of the MI and PKA groups is called for. (One figure and 2 tables of data are included; 18 references are attached.) (KEH)