During the monitoring programme of harmful algal blooms established along the south Atlantic coast of Morocco, a bimonthly determination of harmful algae and phycotoxins analysis in Perna perna was carried out from May 2003 to December 2004. Results of mouse bioassay (in organs and whole flesh) showed a seasonal evolution of paralytic shellfish poisoning (PSP) toxin. The mussel's contamination was associated with the occurrence in water of Alexandrium minutum. The PSP toxin profile obtained with high-performance liquid chromatography (HPLC/FD) revealed the dominance of gonyautoxins GTX2 and GTX3 and a minority of GTX1, GTX4 and saxitoxin (STX). This profile explains that the toxicity was mainly associated with A. minutum.