Résumé : Indicator species with high fidelity to a-priori defined groups of sites are a relevant tool to ecologically characterize plant or animal assemblages. The identification of indicator or diagnostic species is usually performed by summarizing the species abundances within each group of sites. Species with high concentration in a given group of sites are considered diagnostic of that particular group. Among the methods proposed for the determination of indicator species, only very few have considered the species functional traits. This is quite surprising, as species influence ecosystem processes via their traits. Therefore, the species functional traits should give a much better ecological characterization of a group of sites than the species abundances. The aim of this paper is thus to use the species functional characteristics to improve their diagnostic value. These characteristics include the species functional traits and all species-level indicators of environmental association. The proposed method consists of combining the species abundances and their functional characteristics into a single composite index, which can be interpreted as the species fuzzy degree of compatibility with each group of sites. The interpretation of this index in terms of fuzzy set theory allows to introduce a high degree of flexibility in the computation of the species diagnostic values. To show the behavior of the proposed index, two worked examples with data on Alpine vegetation in northern Italy and urban alien species in the city of Brussels (Belgium) are used.