This study investigated developmental changes in memory performance for two contrasting populations in Urban and Rural Yucatan, Mexico. Subjects were divided into five groups defined by age, including children and adults. All urban S's were in school, while only the two younger rural groups were in school, and older rural S's had little or no schooling. The S's were tested on a short-term and incidental memory task. Developmental trends in memory performance for the Urban educated S's were very similar to those reported for analogous tasks with American middle class subjects. The performance of rural S's did not show these consistent developmental changes. This fact, along with the results of several features of the data, support the hypothesis that formal schooling is an important factor in the development of mnemonic skills in short-term memory. Data from the incidental memory task implied that the development of selective attention is independent of short-term memory development and is probably influenced by both school and certain cultural factors. (DP)