Summary.Differences in attainment in English and arithmetic between primary schools in mixed‐language areas of Wales are studied with respect to differences in both intelligence and linguistic background. School attainments tended to increase with intelligence, though not regularly; and the school differences in intelligence did not account fully for the differences in attainment. Generally, the attainment of schools with pupils of a strongly Welsh background—i.e., bilingual pupils—was lower than that of other schools, this tendency being strong in English and slight in arithmetic. It is concluded that a bilingual environment may be regarded as an important factor with respect to primary‐school differences of attainment in English which cannot be accounted for by intel