132 undergraduates were given sequences of 6 to 10 digits presented at 1 digit per sec. with ordered recall instructions. Ss were instructed to rehearse silently in nonoverlapping groups of 1, 2, 3, 4, or 5 digits. Rehearsing in 3's was optimal, being superior to 2's in ordered recall (< .01), item recall (p < .01), and position recall (p < .05), insignificantly superior to 4's in ordered and position recall, but not item recall, and significantly superior to 5's by ordered and position recall (p < .01), but not by item recall. Errors in positioning digits tended to the same position in different groups for groups rehearsing by 2's and 3's (p < .01) and to other positions in the same group for groups rehearsing by 4's and 5's (p < .05). The results support the hypothesis that only 3 serial-position concepts (beginning, middle, and end) are important cues in short-term memory. (PsycINFO Database Record (c) 2006 APA, all rights reserved)