Metacognition involves conscious thinking about one's learning and is implemented in language learning when learners set goals and evaluate their performance, plan their studies, use their language knowledge, attend to the input, search speaking and reading opportunities and ways of enhancing their learning outcomes and focus on their errors. This study sought to measure the use of metacognitive strategies by eight groups of Iranian ESP freshmen in four different disciplines and to detect probable discipline and gender variations. Having assessed the initial homogeneity of the groups via Analysis of Variance of the scores obtained from a Key English Test (KET), the researcher administered the Metacognitive section of Oxford's Strategy Inventory for Language Learning (SILL) (1985). The Between-subjects Analysis of Variance (ANOVA) of the research data revealed that the participants studying Mechanical Engineering and Computer outperformed those studying Management and Psychology.