A study examined the spelling of reading disabled (RD) and average ability students within the context of the developmental lag hypothesis, which was then contrasted with a developmental difference hypothesis. Subjects were 38 regular class third graders and 18 average intelligence fifth graders receiving resource room assistance for reading difficulties. Third grade regular class students were matched on spelling with fifth grade reading disabled students (achieving two years below grade level); average and below average spellers were identified in each class. As part of a larger study examining the relationship between cognitive processing and reading ability, subjects were administered achievement tests, cognitive processing tests, and a computerized spelling test. Hypothesized underlying cognitive processes of memory access, memory span, simultaneous and successive processing, and phonological processing were examined and related to spelling performance. In addition, spelling errors made by the different groups on words differing in word familiarity and spelling predictability were analyzed using a developmental model of spelling. Results showed (to a limited extent) that the underlying cognitive processing supported the developmental lag hypothesis. The analysis of spelling errors much more strongly supported the notion that poor spellers were learning to spell according to a normal developmental pattern but at a much slower rate. In addition, the patterns identified through the analysis of spelling errors provided better diagnostic and instructional information than the analysis of underlying cognitive processes. (Thirty-three references and six tables of data are attached.) (RAE)