The ROM memory cell, not decodable by visual inspection comprises a substrate of semiconductor material having a first conductivity type, in particular P-. A first MOS device having a first threshold voltage is formed in a first portion of the substrate, and a MOS device having a second threshold voltage, greater than the first threshold voltage, is formed in a second portion of the substrate adjacent to the first portion. The second MOS device is a diode reverse biased during a reading phase of the ROM memory cell and comprises a source region having the first conductivity type and a drain region having a second conductivity type. The source region has a level of doping higher than that of the substrate.