International comparative educational studies make use of test instruments originally developed in English by international panels of experts, but that are ultimately administered in the language of instruction of the students. The comparability of the different language versions of these assessments is a critical issue in validating the comparative inferences drawn from the test results. This study analyzed data from the 1999 Third International Mathematics and Science Study (TIMSS) science assessment to evaluation the consistency of the structure of the item response data across different test versions. Individual differences multidimensional scaling analyses were used to evaluate data structure. The findings suggest that slight structural differences exist across countries, and that these differences are related to differences in item difficulty. The implications of these findings for better understanding of international comparisons of educational achievement, and for future research in this area, are discussed. (Contains 1 figure, 7 tables, and 26 references.) (Author/SLD)