Phytoplankton abundance, species composition and primary production of the Gulf of Naples were investigated during an autumn bloom in November 1985. Hydrographic data and surface phytoplankton samples were collected during a 3 day cruise, whereas investigations on in situ primary production and phytoplankton vertical distribution were conducted from a second boat on three different dates. A coast-offshore gradient was recorded for most of the chemical and biological parameters analysed. At stations closer to the coast, which were affected by land run-off, phytoplankton populations attained concentrations of 2.5 x 10(6) cells l-1, with a marked dominance of diatoms belonging to the genera Thalassiosira and Chaetoceros. The most striking character of the system was a remarkably high carbon assimilation rate (up to 1.04 g C m-2 day-1) at stations closer to the coast. The causative mechanism for this bloom appeared to be land-derived nutrient enrichment, possibly enhanced by autumn rains, followed by a period of favourable meteorological conditions, which occurs recurrently in the region for a brief period around November, known locally at `St Martin's Summer'. We hypothesize that a similar mechanism can stimulate phytoplankton growth more than once every year, since high-stability periods following meteorological perturbations can occur several times in temperate regions of the northern hemisphere in autumn.