The professional development of and language integrated learning (CLIL) continues to be a niche in the language education literature, particularly in Latin America. The aim of this study is to explore the effects of a continuing professional development course delivered in 2018 to support language-driven CLIL implementation in state secondary schools in Argentina. Built as a case study, the investigation draws on data from one teacher collected through multiple sources for triangulation purposes. Two research questions guided the study: (1) What beliefs does a teacher have of language-driven CLIL? and (2) What are the effects of a continuing professional development course on language-driven CLIL targeted at EFL secondary school teachers? Findings show that authenticity, rather than language-content integration, is a salient feature of language-driven CLIL in this teacher's practice. Findings also reveal that CLIL success depends on learners' English language proficiency. Drawing on its teacher's beliefs of CLIL, the course had an impact on the teacher's identity as a materials developer and autonomous and critical practitioner who could theorize by reflecting on her practices and the readings provided by the course tutor.