Tracking performance of Monolithic Active Pixel Sensors (MAPS) developed at IPHC (Turchetta, et al., 2001) 1 have been extensively studied (Winter, et al., 2001; Gornushkin, et al., 2002) 2,3. Numerous sensor prototypes, called MIMOSA, 1 1 Standing for Minimum Ionising particle MOS Active pixel sensors. were fabricated and tested since 1999 in order to optimise the charge collection efficiency and power dissipation, to minimise the noise and to increase the readout speed. The radiation tolerance was also investigated. The highest fluence tolerable for a 10 μ m pitch device was found to be ∼ 10 13 n eq / cm 2 , while it was only 2 × 10 12 n eq / cm 2 for a 20 μ m pitch device. The purpose of this paper is to show that the tolerance to non-ionising radiation may be extended up to O(10 14) n eq/cm 2. This goal relies on a fabrication process featuring a 15 μ m thin, high resistivity ( ∼ 1 k Ω cm ) epitaxial layer. A sensor prototype (MIMOSA-25) was fabricated in this process to explore its detection performance. The depletion depth of the epitaxial layer at standard CMOS voltages ( < 5 V ) is similar to the layer thickness. Measurements with m.i.p.s 2 2 Standing for minimum ionising particle. show that the charge collected in the seed pixel is at least twice larger for the depleted epitaxial layer than for the undepleted one, translating into a signal-to-noise ratio (SNR) of ∼ 50 . Tests after irradiation have shown that this excellent performance is maintained up to the highest fluence considered ( 3 × 10 13 n eq / cm 2 ) , making evidence of a significant extension of the radiation tolerance limits of MAPS.