This paper presents the design, implementation, and evaluation of a distributed collaborative UML modelling environment, CoLeMo. CoLeMo is designed for students studying UML modelling. It can also be used as a platform for collaborative design of software. We conducted formative evaluations and a summative evaluation to improve the environment and discover how users collaborate through the environment. We received positive feedback from the evaluations. The findings indicate that this environment is both informative and helpful, especially for less experienced UML modelling users. We also found that less experienced users can learn from their more experienced collaborators. (Contains 1 table and 5 figures.)