This study examined quantitative (dry weight, chlorophyll a content and C : N ratio) and qualitative (community compositions of bacteria and diatoms) changes in marine biofilms as a function of season (summer 2003 and winter 2004), temperature (16, 23 and 30°C) and salinity (20‰, 27‰ and 34‰) under laboratory conditions. Biofilms were allowed to develop for 20 days in the laboratory, using natural sea water collected from Port Shelter, Hong Kong. The following results were obtained: (1) biofilm dry weight was greater in summer than in winter, and greater at 34‰ than at 20‰; (2) biofilm chlorophyll a content was affected by all three factors (season, temperature and salinity), with significant interactive effects among the three factors; and (3) C : N ratio was affected by season (winter > summer) and temperature (30°C>16°C in summer), but not by salinity. Bacterial community composition was analyzed by terminal restriction-fragment length polymorphism of polymerase chain reaction-amplified 16 S rRNA genes. In summer, community compositions of both bacteria and diatoms were strongly affected by salinity. In addition, natural summer biofilms that developed at three field sites, where different salinities were found, harbored appreciably different bacterial and diatom community compositions. In contrast, in winter, temperature exerted a major influence on community compositions. The present study adds to the growing evidence that environmental factors are important determinants of both the quality and quantity of marine biofilms.