We report on experiments studying the appearance of negative ions during ablation from nominally transparent dielectric crystals (Al2O3, BaF2, NaCl), irradiated by an 800 nm, 100 fs-laser in the multiple pulse regime. A state-of-the-art time-of-flight mass spectrometer was used for detection, in combination with ion trajectory simulations. Based on the investigation of the appearance threshold dependence on laser intensity, the kinetic energy and the angular distribution, we present evidence that the expulsion process for negative ions is different from that for positive ions. It appears to be most likely that gas-phase interactions within the ablation plume lead to the creation of negative ions from neutral precursors. (C) 2002 Elsevier Science B.V. All rights reserved.