This study tested the diverging predictions of recent theories of children's learning of spelling regularities. We asked younger (Grades 1 and 2) and older (Grades 3 and 4) elementary school-aged children to choose the correct endings for words that varied in their morphological structure. We tested the impacts of semantic frequency by including three types of words ending in "-er", derived and inflected forms, the first of which are far more frequent across the language, and one-morpheme control forms. Both younger and older children were more likely to choose the correct ending for derived forms over one-morpheme control words. This difference emerged for inflected forms only for the older children. We also tested the impacts of orthographic frequency by contrasting performance on the two derived allomorphs "-er" and "-or", the first of which is far more frequent, and comparison one-morpheme forms. Both younger and older children were more likely to choose the correct spelling for the derivational "-er" over the same letter pattern in control words. This difference did not emerge in either group for the "-or" ending. The implications of these findings for current models of children's spelling development are discussed.