This paper describes the design of a unit selection speech synthesis system for Fon language, a tonal language spoken in Benin. It shows the approach that is used to deal with a text written in Fon language, with steps such as the use of letter-to-sound rules, phonetic and post lexical rules for automatic phonetic transcription of input text. We provide an overview of the implementation of the modules of the system in Java programming language and the various stages of building a synthetic voice in Fon using Festival, a text-to-speech system. The unit selection algorithm used is Multisyn. The results of tests are also discussed.