In recent years, vocabulary has risen up as a key issue in Second Language Learning (cf. Jiménez Catalán & Terrazas Gallego, 2005; Siriwan, 2007; Boers & Lindstromberg, 2008; Schmitt, 2008, 2010). Moreover, CLIL, the acronym for Content and Language Integrated Learning, as an educational approach in which content subjects are taught through a foreign language, has seen a surge in the different educational systems all throughout Europe. The present piece of research aims to present a study about the use of vocabulary learning strategies and their relation to the receptive vocabulary size of secondary school learners. In order to explore this issue, two groups (one CLIL and one non-CLIL) of students from Extremadura were asked to answer two different questionnaires: a Yes/No test (Meara, 2010 [1992]) and a vocabulary learning strategies test (adapted from Schmitt, 1997). The data obtained was analyzed with two specific objectives in mind: to look for differences between the learner groups regarding their use of strategies and between their levels of receptive vocabulary. Results show differences between both groups not only in their receptive vocabulary size, but also in their use of vocabulary learning strategies. From the results of this study, it can be concluded that CLIL instruction seems to benefit the acquisition of foreign language and may also have an influence on the use of certain vocabulary learning strategies.