This paper evaluates the representational nature (as opposed to technical merits) of object-orientation, particularly its strengths and weaknesses, in the context of modelling mobile objects. The evaluation is important amidst the development of object-oriented GIS given that the object-orientation paradigm is not grounded in spatial disciplines. The discussion is illustrated by a case study that uses an object-oriented GIS framework to model the individual fish movement and growth in a heterogeneous aquatic environment.