The aim of the study was to explore how well sixth-graders comprehend the textbooks used in history and natural sciences classes. 247 students took part in the study. Half of them read a text in their natural science book and the other half a text in their history book. After that, they each answered ten questions and explained ten words underlined in the text. The students had no difficulties in answering those questions to which the answers were directly found in the texts. However, questions requiring inference skills proved to be difficult. Explaining the meaning of words was an even more demanding task. It appears that there are many students who enter secondary school with poor reading comprehension skills. Therefore, students should be taught to comprehend texts related to different school subjects and strategies that help them to derive the meanings of unfamiliar words from textual contexts.