Students' difficulty in learning and suitably understanding the concept of the algebraic variable has been studied with a number of tools and documented for several populations. Little research has been conducted, however, using the same tool to explore understanding of the notion among populations from different countries in an attempt to establish similarities and differences in levels of achievement and difficulties. This article discusses the findings of a survey applying a questionnaire designed for use with the 3UV model, a theoretical-methodological tool for analysing the understanding of algebraic variables and their various usages (as specific unknown, as general number, functional). Comparisons were drawn of the results of a survey of a population comprising 184 ninth-year students, 92 in Spain and 92 in Mexico, and 82 Mexican and 85 Spanish eleventh-year students, all from medium-lower income families and attending public schools. The results obtained revealed similarities and differences among the groups, providing evidence of the strengths and weaknesses of the education system of each country.