Achievement in literacy and numeracy by Australian 14-year-olds was examined by analyzing data from the following studies conducted between 1975 and 1998: the Australian Studies in School Performance; the Australian Studies of Student Performance; the Youth in Transition Survey; and the Longitudinal Surveys of Australian Youth. During the study period, mean scores on achievement on tests of reading and mathematics remained stable, and there was little change in the distribution of scores. Female 14-year-olds scored higher in reading comprehension in 1998 than in 1975, whereas their male counterparts scored lower in 1998 than in 1975. The reading comprehension achievement scores of students whose main language was not English improved significantly. Male students' mean mathematics achievement test score increased during the study period, whereas female students' mean score did not. The mathematics scores of students with parents in the production/laborers group remained stable, whereas those of students with parents from the professional/managerial occupational group declined significantly. Being from a metropolitan versus nonmetropolitan area did not significantly affect reading comprehension or mathematics test scores. (Thirty-five tables/figures are included. The bibliography lists 50 references. The following items are appended: counts of students; results of multivariate analyses for all cohorts; and a note on indigenous Australian students.) (MN)