A study used a primed lexical decision task to explore readers' use of "onset" and "rime" units in processing words. Onset segments include the initial consonant or consonant cluster of a syllable, while rime segments consist of the pronounced vowel group and any final consonants. Subjects were 32 fifth graders and 27 college students. Words or word fragments having three relationships to the targets (matching the initial consonants and the vowels, matching the "rime," or unrelated) were used to prime word and nonword targets. Results indicated that: (1) in adults, fragments matching the "rime" facilitated processing while word primes that matched the target in any way interfered with processing; (2) fifth graders showed a similar pattern of facilitation for "rime" fragments but not for "rime" matching words; and (3) fifth graders showed facilitation for word fragments that matched the initial consonants and vowels. Findings suggest that for adults the "onset"/"rime" division is a natural one used in reading and that facilitation from "rime" fragment primes was not caused by a lexical analogy mechanism. Findings also suggest that fifth graders were using units smaller than the "onset" and "rime" or that they are inconsistent in their division of the syllable into subunits. (Three graphs of data are included and 14 references are attached.) (RS)