National standardization data were used to reexamine the evidence of gender differences in achievement test scores reported in previous studies. Changes in differences over time, from 1963 to 1992, were examined, as were differences across all grade and achievement levels. Subjects participated in Iowa Tests of Basic Skills (ITBS), Iowa Tests of Educational Development (ITED), and Tests of Achievement and Proficiency (TAP) national standardizations from 1963 to 1992. Results generally agree with previous studies. In general, the nature and magnitude of differences in achievement were similar over the years. Average differences in achievement between males and females were relatively small in most subject areas, with females scoring higher in reading, language skills, and mathematics computation up to age 15 when the opposite is true in mathematics. Females generally scored higher in verbal tests at the upper grade levels. Male performance was generally more variable than that of females, and these differences in variability lead to interaction between ability level and differences in achievement for males. Males at average and below-average abilities have markedly lower achievement than females. Suggestion are made for future research into language deficits of low-achieving boys. Five tables present study results. (Contains 15 references.) (SLD)