In the present study we investigated a developmentally changing role of text reading fluency in mediating the relations of word reading fluency and listening comprehension to reading comprehension. We addressed this question by using longitudinal data from Grades 1 to 4 and employing structural equation models. Results showed that the role of text reading fluency changes over time as children’s reading proficiency develops. In the beginning phase of reading development (Grade 1), text reading fluency was not independently related to reading comprehension over and above word reading fluency and listening comprehension. In Grades 2 to 4, however, text reading fluency completely mediated the relation between word reading fluency and reading comprehension, whereas it partially mediated the relation between listening comprehension and reading comprehension. These results suggest that text reading fluency is a dissociable construct that plays a developmentally changing role in reading acquisition.