The secretory phospholipase A2-alpha from Arabidopsis thaliana (AtsPLA2-alpha), being one of the first plant sPLA2s obtained in purified state, has been characterised with respect to substrate preference and optimum conditions of catalysis. The optima of pH, temperature, and calcium concentration were similar to the parameters of secretory PLA2s from animals. However, substrate preferences markedly differed. In contrast to pancreatic PLA2s, AtsPLA2-alpha preferred zwitterionic phospholipids, and showed lower activity toward anionic phospholipids. In substrates with two identical fatty acid chains, AtsPLA2-alpha showed optimum activity toward phospholipids with decanoyl groups. In substrates with palmitoyl groups in sn-1 position, acyl chains with higher degree of unsaturation in sn-2 position were preferred, excluding arachidonic acid, showing the evolutionary adaptation of the enzyme to substrate composition in plants. Km values for short chain phospholipids were comparable to sPLA2s from animals, whereas k cat values were much smaller and interfacial activation was less important.