This paper reports on findings of an ongoing study that examines the impact of the Primary School Mathematics and Science Lead Teacher Development Project on the Grade 4 Mathematics classroom learning environments of the lead teachers and their colleagues in cluster schools at Eshowe District. This lead teacher development project responds to lack of skills among primary school mathematics and science teachers, inappropriate models of In-Service Teacher Education and Training (INSET), little teacher development by way of INSET organized by districts, and poor performance in a mathematics and science by students which results from the lack of basic skills in these subjects. The main aim of the project is to establish a clustered-based model of INSET in a district, which is led by a core of primary school lead teachers who are able to demonstrate effective ways of teaching mathematics and science in the intermediate phase (Grade 4 – 6) and to support second-generation teachers in the development of similar skills to convert their traditional classrooms into constructivist classrooms. The base line study results obtained indicated that rote learning, teacher talk and low level questions dominated lessons. The project intervention generally made a good impact on mathematics classroom learning environments in respect of teacher support, involvement, investigation, task orientation and co-operation. However, student cohesiveness needs special attention as it is below the acceptable level.