A silicon substrate (SOI) is placed on a buried oxide layer (BOX). An MOS transistor is produced in an active zone of the substrate which is defined by an isolating region. A gate region and source and drain regions, which between them define a channel, are produced so that the gate region extends above the channel. The isolating region is produced by localized formation of a zone of material that can be selectively etched with respect to silicon. That material is selectively etched, and a dielectric material is deposited in the etched feature. The etching is carried out after the gate region has been produced.