Stability of the mobility-lifetime product of holes has been studied by measuring the ambipolar diffusion length (L a m b ) in undoped a-Si:H films. After 80 hours of AM1 light soaking L a m b decreases from 217 to 152 nm, whereas photoconductivity excited by a HeNe laser with an intensity of 1.7 mW/cm 2 decreases from 7.12 [times ] 10 [ m i n u s ] 6 to 2 [times ] 10 [ m i n u s ] 7 [Omega ] [ m i n u s ] 1 cm [ m i n u s ] 1 . Furthermore, L a m b does not decrease in the first 10 minutes. These results are attributed to the asymmetric defect density (N d ) dependence of the mobility-lifetime products of electrons and holes due to the inherent asymmetry between the density-of-states distributions of the conduction and valence band tail states. Numerical simulation shows that the recombination path via the band tail states dominates over that via the defects when N d [lt ] 3 [times ] 10 1 6 cm [ m i n u s ] 3 .