Interactions between algae and associated bacterial communities in the phycosphere depend on many factors such as culture age, nutrient availability, and antibiotic production and greatly influence algal survival and growth. The microbial community associated with the marine microalga Tetraselmis suecica F&M-M33 in a laboratory culture and an outdoor mass culture, set up from the laboratory one, run during a whole year was investigated in different seasons through isolation in pure culture of bacteria, amplified ribosomal DNA restriction analysis (ARDRA) of all the isolates, and terminal restriction fragment length polymorphism (T-RFLP) analysis. The total number of bacterial isolates was 152, which clustered in thirty-six 16S rDNA groups by ARDRA. Sequencing of a representative of each ARDRA group permitted identification of bacterial genera never reported before in association with microalgae in outdoor mass cultures, although most of them were previously found to be associated with the marine environment (e.g., seawater, sea sediments). T-RFLP analysis revealed that spring and autumn bacterial community profiles were closely related, while the bacterial laboratory community was considerably different (less than 50 % similarity) from that associated with the outdoor culture in different seasons. T-RFLP results suggest the presence of a core of bacteria that are closely associated with the alga and of a part of the community which varies seasonally.