The production of phytoalexins in Thellungiella halophila exposed to UV-radiation and NaCl was investigated over a period of 8 days. UV-radiation induced substantially larger quantities of wasalexins A and B than NaCl irrigation or CuCl(2) spray. Isolation of two metabolites and their chemical structure determination using X-ray diffraction analysis provided the phytoalexins biswasalexins A1 and A2, that resulted from head-to-tail photodimerization of wasalexin A. The production of biswasalexins A1 and A2 in stressed T. halophila, as well as their chemical synthesis and antifungal activity are reported. Biswasalexins A1 and A2 (60 nmol/g and 15 nmol/g fresh wt, respectively, 2 days after UV elicitation) are cruciferous phytoalexins whose formation in planta appears to result from a photochemical reaction, which might protect the plant from fungal attack and UV-radiation.