The problem. This is a study of two bilingual communities in Northern Senegal. In these rural communities some level of bilingual competence in the two languages in contact was expected. The languages are Wolof and Pulaar (a dialect of Fula). Wolof is the subjects\u27 first language and Pulaar is the local majority language. My primary interest was in intrasentential code-mixing. Methods. Fieldwork was carried out in the two areas during the summers of 1987 and 1988. Twenty-five (25) hours of recorded speech were obtained from individual and group interviews and other forms of recordings. The data was later statistically analyzed. Results. The results reveal unexpected patterns of speech. There was a clear-cut difference between the young and the old people. Whereas the older people mix primarily Wolof and Pulaar, as expected, the young have hardly any knowledge of Pulaar. Instead, the young mix Wolof and French, the colonial language, and presently the sole official language of Senegal. The type of language mixing practiced by the two groups is also different. While the old people\u27s speech contains, by and large, loanwords from Pulaar, the speech of the young contains all kinds of code-mixing. In the young people\u27s speech, I was able to find solid evidence for the existence of nonce borrowing called in the present work lexical-switching or synchronic borrowing. The importance of this finding is that it invalidates certain claims regarding the violation of Pfaff\u27s (1979) structural constraints as well as Sankoff et al\u27s (1980) free morpheme constraints and equivalence constraint. It also sheds new light on the distinction between code-switching and nonce-borrowing. Conclusions. Two main conclusions are drawn from the results. First, the differences noticed between the old and the young are a result of a process that dates back to the time of the colonization of the country by France. Colonization established the French language in the country and ensured that it would occupy the official domain. This position has given the French language a guarantee of survival even after independence. Paradoxically, colonization also gave a boost to the Wolof language whose speakers were the first to enter in contact with the colonial masters. Second, the progressive loss of Pulaar among the young people in the community is the direct result of the policies, including linguistic policies, that started with colonialism and were carried on after independence. As Wolof and French gained prestige nation-wide, Pulaar lost its local prestige among these people