Although there is a lot of interest in the development of computational thinking (CT) and the benefits it could have for every student, integrating it into science classrooms may be more difficult than traditional teaching. This can be very challenging for South African science teachers. Thus, there is an increasing need to prepare teachers and systematically engage them in professional development activities to help them learn, unlearn, relearn and restructure their thoughts, perceptions and attitudes towards integrating CT in science classrooms. This study explored teachers' perceptions and concerns about integrating CT in science teaching. Twenty-two science teachers from innercity secondary schools in South Africa were chosen to be part of this exploratory case study. Data were collected through take-home assignments and focus group discussions containing a series of open-ended questions. The concern-based adoption model was used as a lens to understand teachers' stages of concern. Data were analyzed using thematic analysis. It was found that sampled science teachers viewed CT as a valuable tool that can help promote learners' data management practices and problem-solving practices, especially in reducing complex problems to researchable problems. The findings also showed that teachers intend to integrate CT into their science classrooms by refining their existing practices. However, teachers showed high informational, personal and management-related concerns regarding integrating CT in South African science classrooms: they needed more practical and contextual ideas on strategically integrating CT in their daily lessons. Conversely, teachers showed limited concern at the unrelated, consequence, collaboration and refocusing stage. This suggests a need for teacher education programmes in South Africa to focus more acutely on training pre-service teachers on how to plan and design science instruction using computational thinking concepts and practices.