The objective of the current study is to investigate the development of awareness of dialectal differences in Spanish by second language learners in a formal language learning classroom setting in the United States. Sensitivity toward different regional varieties of a language forms part of socio-linguistic--and ultimately, communicative--competence (Bachman, 1990; Canale and Swain 1980; Canale 1983), a primary objective of language learning. However, the role of dialect awareness in second language learning is largely not understood. 229 American English-speaking learners of Spanish across five proficiency levels of university study completed a questionnaire that elicited knowledge of dialectal phonetic variation in Spanish. Results found that, while a variety of dialectal pronunciation features and regions were identified, learners were most aware of dialectal features of Peninsular (Spain) Spanish. However, as proficiency level increased, learners became more knowledgeable of dialectal differences in pronunciation throughout the Spanish-speaking world and reported explicit knowledge of a greater variety of dialectal structures and regions. Patterns in the development of dialect awareness in a second language are identified, and implications of dialect awareness for language learning and teaching are explored.