Copyright © 2003 IFAC. This paper describes an active buoyancy control system for an autonomous underwater vehicle (AUV). The AUV 'Tethra" is a shallow-water craft designed for a target application of underwater video surveying and as a general AUV test-bed. The craft is controlled using an enhanced version of Brook's behaviour based Subsumption Architecture. Mission scenarios that might require active buoyancy control have been identified. Active buoyancy control is realised using ballast tanks and compressed air. A secondary emergency controller and an Emergency Buoyancy System (EBS) are also presented. Buoyancy control behaviours have been developed and are incorporated in the craft's autonomous control algorithms.