The detail design of AUTOSUB-1 began in March 1995 following a period of sub-system development; in the mechanical field, this proved the practicability of composite pressure vessels, direct drive propulsion and fin control systems. Complete assemblies were tested in the laboratory and in the Southampton Oceanography Centre's pressure vessel, simulating, as far as was practicable, the working conditions. The integration of these and other systems into a complete vehicle was advanced sufficiently over a 15-month period that sea trials could begin. The sea trials from June 1996 to April 1997 have been the ultimate proof of reliability and performance, and have demonstrated the overall performance of the vehicle. This paper describes the mechanical aspects of the main pressure vessel, propulsion system, fin actuation, emergency abort, scientific payloads, and the subsequent performance obtained at sea.