A mounting frame for an electronics and instrumentation enclosure includes two elongated supports joined by an elongated piece and four corner parts fastened to the ends of the elongated supports by a fastener. For the mounting frame to be easy and rapid to assemble and to mount inside the enclosure also at the mounting site, the ends of the elongated supports are provided with a locking tongue formed as one piece with the elongated support and extending away from a plane defined by the elongated supports, and through-hole for mounting are provided at the corner parts and arranged to receive the locking tongues of the elongated supports without clearance, the corner parts comprising a plural number of such mounting holes at a distance from one another in the longitudinal direction of the corner parts. An elongated support is provided for a mounting frame.