This paper reports on a study that employed metacognitive and sociocultural theoretical frameworks to examine students' metacognitive regulation in a collaborative problem-solving context. Specifically, we attempted to understand how elementary students' (5th and 6th graders) metacognitive regulation could be activated when students worked collaboratively in small groups in a science classroom, and how metacognitive regulation could assist students' scientific problem solving. To achieve this research purpose, we employed descriptive and interpretive case study. Two groups of four students were selected as focus groups. Both groups included a mix of genders (female and male) and ages (grade 5 and 6). Qualitative data, such as video records of classroom learning and interactions and semistructured interviews, were collected over a three-month research period. Through case analysis of those data, we found that factors such as anomalies in task performances, different ideas appearing during students' collaborative problem-solving activities, and uncertainty concerning these ideas had the potential to activate students' metacognitive regulation. Through further exploration, we also found that students' attitudes towards both their collaboration and different ideas emerging during their group work, as the underlying mechanism for activating metacognitive regulation, could influence whether the aforementioned factors work or not. Moreover, this study demonstrated that students' metacognitive regulation could positively impact many aspects of scientific problem solving.