Students'difficulty in learning and suitably understanding the concept of the algebraicvariable has been studied with a number of tools and documented for severalpopulations. Little research has beenconducted, however, using the same tool to explore understanding of the notionamong populations from different countries in an attempt to establishsimilarities and differences in levels of achievement and difficulties. This article discusses the findings of asurvey applying a questionnaire designed for use with the 3UV model, atheoretical-methodological tool for analysing the understanding of algebraicvariables and their various usages (as specific unknown, as general number,functional). Comparisons were drawn ofthe results of a survey of a population comprising 184 ninth-year students, 92in Spain and 92 in Mexico, and 82 Mexican and 85 Spanish eleventh-yearstudents, 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 ofeach country.