Two studies examined children's misunderstandings in using dictionary definitions to comprehend the meaning of a new word. Subjects in the first study, 38 male and 22 female fourth-grade students from four classes in a medium-sized midwestern town, were given two similar judgment tasks: (1) students were given a definition for a new word and asked whether the definition made sense for the given sentence; and (2) students were given the sentences first and asked whether the definition made sense for the new word in the sentence. Results indicated that the students treated fragments of definitions as representing the meaning of the word defined; and that the tendency to make such errors was essentially the same at all levels of reading ability. Subjects in the second study, 55 fourth-grade students and 45 sixth-grade students from the same school district as the first study, were given definitions in three formats (conventional, conventional definitions plus example sentences, and nonconventional definitions) and asked to decide which definitions made sense. Results of the second study indicated that students in both grades chose inappropriate sentences using fragments of definitions with regularity; and that this type of error was not conditioned by definitional format. (Nine tables and two figures of data are included; 45 references are attached.) (RS)