ABSTRACT. – The goal of this study is to evaluate the influence of four Geneva geotypes (characterized by the composition of trace elements in water), of the season and of water quality on the distribution of epilithic diatoms in the main rivers in the canton of Geneva (Switzerland). To achieve this, we used 72 samples of diatoms as well as some physical-chemical and bacteriological analyses. The sampling was conducted in the framework of monitoring the surface water quality between 1998 and 2004. For each of these 72 samples, the season (winter-spring or summer) as well as one of the four geotypes (“Jura”, “Plaine”, “Alpes”, “Marais”) was accounted for. Nine sample groups based upon the distribution of 120 diatom taxa were defined by statistical analyses. The flora of these nine groups is mainly composed of common and frequentlyoccurring species in the surface waters of the Swiss Plateau that represents good to average water quality. Nonetheless, some of the rarest species confined to poorly-nourished calcareous regions, were sampled only in certain groups. Our results show that in addition to the water quality, the geochemical signature influences the floral composition of Geneva’s diatoms. It would be interesting to expand this type of study to the Swiss hydrographical network, to define its ecoregions, to measure its influence on the assemblage of diatoms and to ultimately adapt it to our objectives of biological quality.