This study investigates the theoretical plausibility and empirical validity of the assumption that all performance in a foreign language can be traced back to a single factor, the general language ability factor. The theoretical background of this hypothesis is reviewed in detail. The concept of a unitary linguistic competence, interpreted as an internalized expectancy grammar of a foreign language learner (Oller), is discussed. What appears in Oller (1976) to be the most convincing evidence for a general ability factor is criticized as a methodological artifact. The data on which the evidence is based are reanalyzed and compared with new research results. It is argued that there is theoretical ground for the assumption of a general language competence as well as more specific skills underlying foreign language learning. The empirical evidence for the existence of a general ability factor and its influence on different modes of language performance is found to be quite convincing but not as clear as Oller asserts. Alternative interpretations for the existence of a strong first factor are taken into consideration. Finally, some implications of the findings for the teaching of a foreign language are discussed. (Author/JB)