This study sheds light on the achievement gap between two culturally diverse populations in Israel by employing a diagnostic model for analysing responses of a representative sample of Jewish and Arab fourth graders on a national mathematics test. The results indicated large significant differences, in favour of the Jewish group, on most attributes underlying the test, and relatively small significant gender effects only in the Jewish group, where boys outscored girls on higher-order thinking attributes. These results were discussed in light of cultural differences between the two populations, educational resources, and prevalent instruction-learning-assessment cultures in their respective schools. (Contains 3 tables and 1 figure.)