This paper presents a real-life pest control expert system for tomato. The system involves two main subtasks, namely: ‘diagnose’ and ‘treat’. The ‘diagnose’ subtask finds out the causes of the growers' complaints, while the ‘treat’ subtask finds out a treatment plan for these causes. CommonKADS methodology has been used to develop the system. Dependency network is used as one of our knowledge representation schemes in both subtasks. An expert system evaluation methodology has been suggested and applied to the developed system. © 2000, Springer-Verlag London Limited.