A study examined longitudinal differences in the academic achievement of economically advantaged and economically disadvantaged pupils of high, average, and low academic ability in grades 2 through 9. Data used were the 1982 and 1985 Stanford Achievement Test Reading Comprehension results of the Palm Beach County, Florida, school district. The pupil population reflected a cross-section of socioeconomic backgrounds ranging from a Palm Beach resort environment to that of the migrant farm worker. Participation in the district's federal lunch program was used as the basis for sorting subjects into economically advantaged and disadvantaged groups. The reading comprehension scores were used to further divide group subjects into high, average, and low achievement subgroups. The results reinforced previous findings indicating that, as a group, economically advantaged students academically outperform economically disadvantaged students. (Tables of data are included.) (HTH)