A multilevel approach was used to examine determinants of growth in grade 8 mathematics achievement in Thailand. Data for 99 mathematics teachers and their 4,030 eighth graders (the 14-year-old cohort) were taken from the Second International Mathematics Study (1981-82) of the International Association for the Assessment of Educational Achievement. The 13 primary sampling units were the 12 national educational regions of Thailand plus the capital, Bangkok. The schools in this sample were equally effective in converting pretest into posttest scores; there were essentially no variable slopes in this respect. When group and individual effects on total variance were examined, group level effects contributed 32% of the variance and individual effects contributed 68% of the variance in posttest scores, with achievement higher for: (1) boys; (2) younger students; (3) children with higher educational aspirations; (4) those with higher self-perceptions of ability; and (5) those with greater interest in and perceived relevance of mathematics. The model developed explained most of the between-school, but less of the within-school, variance. It is suggested that schools in Thailand are more uniform in their effects than previous research in developing countries has suggested. Nine tables present study data. A 33-item list of references is included. (SLD)