Research studies which make comparisons between different ethnic groups frequently assume that cultural patterns coincide with ethnic categories. This assumption often leads to a reliance on cultural explanations for any differences found between ethnic groups. Such explanations may be erroneous in multi-ethnic societies, unless the extent to which members of a particular ethnic group are competent in the language and cultural practices of that ethnic group is assessed. Comparisons of Maori and Pakeha (European) groups in New Zealand are used to illustrate the importance of the distinction between ethnicity and culture. Results are presented which indicate that Maori children with some knowledge of Maori language and culture have higher scores on achievement tests than Maori children who have little or no knowledge of Maori culture. Possible reasons for these results are discussed.