This experimental study examined how the presence of visual diagrams in 2 science texts affected the independent reading comprehension of 213 fourth-grade students in the southeastern United States. In addition, the relative impact of 3 diagram designs and layout (representational, interpretational, or integrated) was compared with a control. Prior knowledge was measured and statistically controlled for each text. Comprehension measures were a term-selection posttest and a reading comprehension posttest. Overall, on both texts, visual diagrams provide minimal or no added value to students' comprehension above the text-only conditions. Findings also indicated that the integrated diagram may create a condition of cognitive overload for some students. Implications for instructional material design are discussed.