This investigation connects issues in second language (L2) acquisition to topics in quantitative sociolinguistics by exploring the relationship between native-speaker (NS) and L2 variation. It is the first large-scale analysis of L2 mood use (the subjunctive-indicative contrast) in Spanish. It applies variationist findings on the range of linguistic and extralinguistic factors (form regularity, semantic category, time reference, hypotheticality, and task) shown to influence NSs' mood use to an interlanguage analysis of L2 development and shows that analyses of frequency and predictors provide revealing details about how learners acquire the ability to vary their use of verbal moods in Spanish. Variationism, it is concluded, can foster rich descriptions and explanations of interlanguage and its evolution.