This document presents Part I of a two-part study which sought to ascertain the relationship of grade level, achievement level, sex, and method of presentation to the various bases by which children classify geometric concepts. Two tasks, administered consecutively to 96 subjects in grades five, eight and eleven, consisted of the sequential presentation of an array of eight geometric concept cards (Task I), and a 26-item picture array of geometric concept instances presented simultaneously (Task II). Responses for Task I were categorized as: Perceptible, Attribute, Nominal, and Subject-fiat. Responses on Task II were categrozied as: Perceptible, Attribute, and Nominal. Essential findings, which are summarized in Part II, were that an increase in grade level was accompanied by decrease in the use of the Perceptible basis of classification and an increase in the Attribute and Nominal; high achievers used the Perceptible category less and the Attribute and Nominal categories more than low achievers; sex differences were not significant; and subjects who were presented with pictorial stimuli gave more Perceptible responses than those presented with verbal stimuli on Task I. For summary, appendices, and references, see CG 006 041. (Author/CJ)