This study investigated how task complexity affected native and non-native speakers' speaking performance in terms of a measure of communicative success (functional adequacy), three types of fluency (breakdown fluency, speed fluency, and repair fluency), and lexical diversity. Participants (208 non-native and 59 native speakers of Dutch) carried out four simple and four complex speaking tasks. Task complexity was found to affect the three types of fluency in different ways, and differently for native and non-native speakers. With respect to lexical complexity, both native and non-native speakers produced a wider range of words in complex tasks compared to simple tasks. Results for functional adequacy revealed that non-native speakers scored higher on simple tasks, whereas native speakers scored higher on complex tasks. We recommend that, in future research examining effects of task types on task performance, the notion of functional adequacy be included. Appended are: (1) Short descriptions of all eight speaking tasks; and (2) Scale for rating responses in the persuasive informal complex task (translation from Dutch original). (Contains 2 tables and 1 footnote.) [Funding for this research was provided by the Netherlands Organisation for Scientific Research. For complete volume, see ED539539.]