The ability to recognize letter patterns within words as a single unit is important for fluent reading. This skill is based on previously established memory representations of common letter patterns. The ability to form these memory representations may be impaired in some poor readers, particularly readers with naming speed deficits (NSD). This study explored factors that influence letter processing and the subsequent formation of memory representations of letter strings in children with and without a NSD. Children were presented with a letter string, followed by a probe unit that was either a single letter, a two-letter cluster, or a repetition of the whole string. Children indicated whether or not the probe had been present in the preceding string. Two factors were manipulated: (a) amount of time to process the initial letter string, and (b) level of orthographic structure present in the letter string. Results indicated that overall, children with NSD performed less accurately than children without NSD. However, children with NSD showed no differential benefit in performance as a result of longer time to process a letter string. In addition, all readers were able to make use of the orthographic structure in a letter string to aid performance. Implications of results for establishing memory representations of letter strings are discussed.