Using a reliable and broad-based measure of morphological awareness, which tapped knowledge of relational, syntactic, and distributional morphology, we examined the development of morphological knowledge among older elementary students and the relationship of their morphological knowledge to a range of literacy measures. We found that morphological awareness continued to develop from fourth to sixth grade and development was most pronounced when derivational forms required phonological changes to their base words (i.e., phonologically opaque items such as signature/sign). Furthermore, children's skill with phonologically opaque items on our morphological assessment made a unique contribution to real-word and non-word reading, beyond phonological awareness and vocabulary. In the case of comprehension, neither morphological nor phonological awareness accounted for unique variance once vocabulary was considered as a factor. (Contains 6 tables.)