This paper reports on the determination of the toxin profile of batch cultures of Alexandrium ostenfeldii established from water samples collected along the Emilia-Romagna coasts (Italy) in November 2003. Combined use of LC-MS/MS and 1D and 2D NMR techniques led us to identify 13,19-didesmethylspirolide C and 13-desmethylspirolide C as products of the alga and to highlight the occurrence of a new component, 27-hydroxy-13,19-didesmethylspirolide C, whose structure was fully elucidated by NMR. MS data for a number of minor spirolides are also reported.