Research on second language acquisition has already become a well-established topic in Europe and America. However, there are only few studies dealing with second language acquisition in Africa despite the fact that major languages like Swahili are mainly acquired as second languages. The study is an investigation of the interlanguage Swahili of speakers who have the Eastern Nilotic language Maa as their first language. They have acquired and are acquiring Swahili informally. Interlanguage is considered to be the result of a creative learning process that has to be analysed as an independent linguistic system. As interlanguage is characterised by high variability the data base consists of several texts of Maa speakers whose linguistic systems represent different levels of grammatical complexity. A functional approach has been used for the linguistic analysis. For each text a grammatical description of its specific linguistic features is provided. Finally, there is a statistic comparison of morphological and syntactic features of all the texts. The conclusion discusses the relationship between pidginization and second language acquisition and attempts to answer the question whether the investigated variety of Swahili can be classified as a pidgin language.