Phospholipid metabolism is involved in plant responses to drought and salinity stress. To investigate the role of phospholipase D (PLD) and its product phosphatidic acid (PtdOH) in stress signalling, we isolated a novel PLD cDNA, designated AtPLDdelta, by screening a cDNA library prepared from dehydrated Arabidopsis thaliana. The AtPLDdelta protein, of 868 amino acids, has a putative catalytic domain and a C2 domain that is involved in Ca2+/phospholipid binding. The AtPLDdelta mRNA accumulated in response to dehydration and high salt stress. Histochemical analysis showed that the AtPLDdelta gene is strongly expressed in the vascular tissues of cotyledons and leaves under dehydration stress conditions. Under normal growth conditions, AtPLDdelta was expressed in roots, leaves, stems and flowers but not in siliques. We showed that dehydration stimulates the accumulation of PtdOH. The accumulation of PtdOH in response to dehydration was significantly suppressed in AtPLDdelta antisense transgenic plants. These results suggest that AtPLDdelta may be involved in PtdOH accumulation in the dehydration stress response.