The main aim of this study was to produce a computer advisory program which could help the farmer choose the most suitable control method against parasitic gastro-intestinal nematodes in sheep. Chapter 2 discusses the biology and epidemiological patterns of the nematode species responsible for causing parasitic gastroenteritis in sheep. Chapter 3 then goes on to describe the anthelmintic drugs currently marketed in the UK for worm control in sheep. Chapter 4 introduces the concept of an expert system, an area of artificial intelligence which has come to the fore over recent years. In Chapter 5, the sheep farm survey carried out in 1992 of sheep farmers in the Gwynedd area of North Wales is discussed. It was shown that the basic trends found in other similar surveys over recent years were also found here. The survey highlighted the need for a new approach to educating the farmer on effective worm control. The development cycle of the expert system application WORMS is then described in Chapters 6 and 7. WORMS was developed using the expert system shell Crystal4. This ultimately led to the production of a user guide and the documentation of four case histories to demonstrate the methods used by WORMS to reach a conclusion. Two auxiliary programs are also included into the WORMS application as educational tools to enhance the farmer's understanding of worm control. These include a database of all products currently marketed in the UK for sheep and how to use these products effectively.