The disclosure relates to a method of simultaneous fabrication of an MOS transistor of SOI type, and of first and second transistors on bulk substrate, comprising: a) providing a semiconductor layer on an insulating layer covering a semiconductor substrate; b) forming a mask comprising, above the location of the second transistor, a central opening which is less wide than the second transistor to be formed; c) plumb with the opening, entirely etching the semiconductor layer and insulating layer, hence resulting in remaining portions of the insulating layer at the location of the second transistor; d) growing the semiconductor by epitaxy as far as the upper level of the semiconductor layer; e) forming isolating trenches; and f) forming the gate insulators of the transistors, the gate insulator of the second transistor comprising at least one part of the said remaining portions of the insulating layer.