Language-learning aptitude and crosslinguistic similarity between learners’ first language (L1) and the target second language (L2) are both known to facilitate successful L2 learning. However, these phenomena have rarely been investigated together in the same study. To address this research gap in second language acquisition, the present study was carried out with 92 international students of Swedish as a L2, with diverse L1 backgrounds. The participants first completed a language aptitude test upon entering a six-week introductory L2 course at the beginning level. Their L1 background was categorized in relation to the target language as either similar (Germanic L1) or distant (non-Germanic L1). At the end of the course, the participants completed a test of L2 achievement. Regression analyses of achievement scores, with language aptitude and L1 background as independent variables, revealed that crosslinguistic similarity explained at least as much variance in L2 achievement as did language aptitude. When comparing the effects of aptitude in the two L1 subsamples, language aptitude was found to be more important for the learners with a typologically similar L1, than for the learners with a more distant L1. In addition, the results provide support to theoretical proposals made in the individual differences literature that indicate that auditory processing ability may be of particular importance in the earliest stages of L2 acquisition.