One of the most intriguing lines of investigation suggested by Foster (2020) refers to the relationship between free vocabulary use and fluency performance measures in speech. The hypothesis of a strong relationship between vocabulary and fluency had already been formulated also regarding writing, since one of the definitions proposed for writing fluency can be summarised as the speed of lexical retrieval while writing (Snellings et al. 2004). Thus, being a fluent writer, at least according to such slightly narrow definition, would be a consequence of being able to quickly access the mental lexicon in order to retrieve the lexical items needed.

The present study investigates the written production in a narrative and a decision-making task by Swedish university students of Spanish as a foreign language. The relationship between lexical richness (diversity and sophistication) and fluency measures is analysed with the aim of finding possible changes due to the different degree of task complexity. The result showed that, when manipulating task complexity in both task types, the relationship between lexical richness and fluency turned out to be unclear, as it varied in unpredictable ways depending on the learners´ overall proficiency and passive vocabulary knowledge. Implications, limitations, and promising future lines of investigation will be discussed in light of previous research.