To determine the frequency of simple main ideas (explicit statements found in a single T-unit), delayed completion main ideas (explicit statements using no more than two T-units), and inferred main ideas in social studies textbooks, 100 passages from second, fourth, sixth, and eighth grade textbooks were analyzed. Results of the study revealed that only 27% of the passages contained simple section main ideas, 44% of all paragraphs contained simple main ideas, and 27% of all paragraphs began with simple main idea statements. Taken collectively, the results suggest that readers of these textbooks must rely heavily on their inferential comprehension ability to identify main ideas. These findings indicate that (1) teachers need to provide students with adequate instruction in comprehending main ideas, especially implicit main ideas; (2) educators and researchers must provide teachers with effective comprehension strategies; and (3) publishers must be more diligent in requiring authors of content textbooks to directly state section and paragraph main ideas. (MM)