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This paper describes a Software Development System (SDS) that was implemented at Bell Laboratories to support the development of software for Electronic Switching Systems (ESS). The SDS supports multiple programmers and multiple releases. It permits incremental development in a timely and efficient manner while maintaining project management control and tracking capabilities. Software testing and debugging tools are also provided. The SDS was implemented and is currently being used to support the development and maintenance of software used in the No. 2B and No. 3 ESS manufactured by Western Electric for the Bell System. There are approximately 500 of these systems currently serving telephone subscribers. This paper describes the SDS implementation for the No. 2B ESS, and discusses some of the user and project experiences.