This article presents the results of a longitudinal study of digital writing strategies and vocabulary development among Swedish upper secondary school pupils of Spanish as a foreign language. During one schoolyear, the pupils (N=31) wrote four essays in Spanish, a pre-test and a post-test. 15 pupils used a printed dictionary as translation tool, 16 used Google Translate (GT). Vocabulary development was measured as the change in lexical diversity from pre-test to post-test, using Guiraud’s index. The results show that the use of GT leads to higher degrees of lexical diversity as long as it is used, but that the effect vanishes when the tool is no longer used. The results point to the need for a deeper understanding of language structure and lexicon, a reinforced focus on vocabulary in foreign language teaching, and a widened range of explicit instruction of translation tools and strategies.