Seawater samples were weekly collected from the Ría of Pontevedra (NW Spain) during 1998 as part of a harmful algae monitoring programme. Phytoplankton cells were identified and enumerated by light microscopy (integrated samples from 0-15 m depth). Pigment composition was determined by HPLC after size-fractionating (GFD and GFF filters). Two major proliferation episodes of Pseudo-nitzschia spp. were detected in March 1998 (400.000 cells L-1), and July 1998 (800.000 cells L-1). During the first maximum, Pseudo-nitzschia spp (50% of total diatom numbers), was mainly dominated by the toxic P. australis. Pigment analysis showed a chlorophyll (Chl) c pattern with Chl c2 as major compound, while Chl c1 and Chl c3 were minor components. Along summer proliferation, Pseudo-nitzschia spp (90% of total diatom numbers), was dominated by the non-toxic P. fraudulenta. Pigment analysis showed Chl c2 and Chl c3 as major components of Chl c family, and low Chl c1concentrations. Although Chl c3 is usually associated with members of the classes Prymnesiophyceae, Pelagophyceae and some species from Dinophyceae, it has also been detected in several Pseudo-nitzschia species as P. fraudulenta, P. delicatissima, P. pungens and P. pseudodelicatissima, but not in P. multiseries and P. australis, species able to synthesise domoic acid, the causative agent of amnesic shellfish poisoning (ASP). Other characteristic feature of HPLC pigment analysis was the detection of a catotenoid with lycopene-type spectrum, just after the decline of Pseudo-nitzschia proliferation. The carotenoid was detected in the GFF (fine fraction) and its potential ecological meaning will be discussed. The parallel increase of Chl c3 values and Pseudo-nitzschia cell numbers (throughout the development of a quasi mono-specific Pseudo-nitzschia spp proliferation) could be used as useful additional information, but obviously, only during the time that the decision makers are waiting for the results of the official assay (i.e. HPLC domoic acid analysis), which imply a certain time-delay while the molluscs are incorporating domoic acid into its tissues.