This article describes the development and validation of an instrument that can be used to assess students' perceptions of their learning environment as a means of monitoring and guiding changes toward social constructivist learning environments. The study used a mixed-method approach with priority given to the quantitative data collection. During the quantitative data collection phase, a new instrument--the "Social Constructivist Learning Environment Survey" (SCLES)--was developed and used to collect data from 1,955 grade 9 science students from 52 classes in 50 schools in the Western Cape province, South Africa. The data were analysed to evaluate the reliability and validity of the new instrument, which assessed six dimensions of the classroom learning environment, namely, "Working with Ideas," "Personal Relevance," "Collaboration," "Critical Voice," "Uncertainty in Science" and "Respect for Difference." Two dimensions were developed specifically for the present study in order to contextualise the questionnaire to the requirements of the new South African curriculum (namely, "Metacognition" and "Respect for Difference"). In the qualitative data collection phase, two case studies were used to investigate whether profiles of class mean scores on the new instrument could provide an accurate and "trustworthy" description of the learning environment of individual science classes. The study makes significant contributions to the field of learning environments in that it is one of the first major studies of its kind in South Africa with a focus on social constructivism and because the instrument developed captures important aspects of the learning environment associated with social constructivism.