The study reported in this paper investigated the relationship between components of aptitude and the fluency, lexical variety, syntactic complexity, and accuracy of performance in two types of written and spoken narrative tasks. We also addressed the question of how narrative performance varies in tasks of different cognitive complexity in the written and spoken modes. Our findings indicate a complex interaction between aptitude components and task performance under different conditions. The components of aptitude that seemed to be most strongly related to the complexity and accuracy of production were inductive ability and grammatical sensitivity. The results also show that in writing the participants used more varied vocabulary than in speech, but their performance was similar in terms of syntactic complexity.