The design of an optical fibre communications network is a very knowledge-rich domain, due to the emerging nature of the art. Such optical fibre systems are used in very large (e.g. global) to very small communications networks, e.g. instrumentation links. This paper describes an expert system, built to fulfil the twin needs of a tutorial advisor as well as a design consultant for the design of small optical-fibre networks. One part of the expert system is a tutorial introduction to the terminology and basic principles of the art of the design of optical fibre networks. The other part is a designer's consultant which requests data from the inexpert user about the requirements of the optical fibre network, and then offers design choices for optical sources, fibres and receivers on the basis of experiential heuristics, as well as well-established mathematical relations. The expert system is implemented using a very cheap (<$Aust 200) expert-system shell which meets most of the needs of this application. The use of a ‘shell’ allowed rapid building of the expert system in a few weeks by an undergraduate student (under academic guidance) starting from complete unfamiliarity with both the expert-system methodology and optical-fibre technology. The induction facility of the expert-system building tool was used to translate device data and design decision tables into rules automatically, thus saving time and avoiding syntactical errors.