This quasi-experimental study reports on a 10-month classroom-based longitudinal investigation, exploring the potential of authentic materials to develop Japanese learners' communicative competence in English. Sixty-two second-year university students were assigned to either a control group receiving textbook input or an experimental group receiving authentic input, and their pretreatment and posttreatment levels of overall communicative competence were assessed. Communicative competence was operationalized with a batch of eight different tests: a listening test, a pronunciation test, a C-test, a grammar test, a vocabulary test, a discourse completion task, an oral interview, and a student-student role-play. The results indicated that the experimental group outperformed the control group in five of the eight measures, suggesting that the authentic materials and their associated tasks were more effective in developing a broader range of communicative competencies in learners than the textbook materials. I discuss the pedagogical implications of these findings for language teachers and their learners.