Background: The use of cognitive tests is increasing in Africa but little is known about how such tests are affected by the great ethnic and linguistic diversity on the continent. Aim: To assess ethnic and linguistic group differences in cognitive test performance in the West African country of the Gambia and to investigate the sources of these differences. Samples: Study 1 included 579 participants aged 14-19 years from the Wolof and Mandinka ethnic groups of the Gambia. Study 2 included 41 participants aged 12-18 years from the two ethnic groups. Methods: Study 1 assessed performance on six cognitive tests. Participants were also asked about their history of education, residence in the city, parental education, and family socio-economic status. Study 2 assessed performance on two versions of the digit span test. Recall of the numbers 1-5 were compared with recall of numbers 1-9 for both the Wolof (who count in base 5) and the Mandinka (who count in base 10). Results: Study 1 established that Wolof performance was lower than that of the Mandinka on five out of six cognitive tests. In four of these tests, group differences were partially mediated by participation in primary school and migration to the city. Group differences were substantial for the digit span test and were not attenuated by mediating variables. Study 2 found that digit span among the Wolof was shorter than that of the Mandinka for numbers 1-9 but not for numbers 1-5. Conclusions: Several suggestions are made on how to consider the ethnicity, language, education, and residence (urban vs. rural) of groups when conducting comparative cognitive assessments or collecting normative data.