A phytoplankton data series, regularly collected at a coastal monitoring station since 1984 and cyst surveys from sediments indicate that several potentially toxic species are present in the Gulf of Naples. These include species of the genus Alexandrium (e.g. A. tamarense, A. minutum, A. andersoni), Dinophysis (D. sacculus, D. fortii, D. caudata, D. rotundata), Protoceratium reticulatum, Gymnodinium mikimotoi, G. cfr. catenatum, Chattonella subsalsa, Pseudo-nitzschia delicatissima and P. pseudodelicatissima. Concentrations may be very high (up to 4x106 cells l-1) in the case of Pseudo-nitzschia species at the long-term station. Other species, such as Gymnodinium mikimotoi, P. reticulatum and C. subsalsa are recorded sporadically and only from net samples. Still others, such as Alexandrium andersonii and G. cfr. catenatum have never been recorded in plankton samples but have recently been found during the course of a cyst survey in the area. This shows the advantage of integrating different sampling methods in order to have a more complete list of HAB species which are present in a given site. In the inner part of the Gulf there are several mussel cultivation farms, yet intoxication events attributable to harmful algae have never been reported in the area. We discuss the role of threshold concentrations of harmful species, intraspecific variability in toxicity, and oceanographic characteristics of the area among the possible reasons for the apparent lack of a negative impact on mussel cultivation and human health.