The macrobenthic communities of many river basins of Central Italy were sampled and analysed with the Extended Biotic Index, modified for Italian rivers (I.B.E.); 101 watercourses of various typologies were sampled and the total number of taxa detected was 130, among which 117 were useful for the determination of the I.B.E. A preliminary analysis was performed in order to determine the correlation between the frequencies of the different Systematic Units (S.U.) and the I.B.E. values. The data were processed in order to develop an index, the Survey Frequency Index (S.F.I.), which computes the presence of a single S.U. at a given I.B.E. score, in relation to the number of sampling, sites showing the same score (I = Ay/nBy). The presence of a faunal group - Index of Group Presence (I.G.P) - and of taxon - Index of Taxon Presence (I.T.P.) - was evaluated estimating the average of the mean families and the taxon presence respectively. The comparison between the autoecology of a given taxon and its relative I.B.E. and Quality Class (Q.C.) index value provides a preliminary interpretation of a reference community structure within a watershed. The analysis of the macrobenthic community has revealed abnormal distributions of some taxa in relation to the water quality in all of the considered river bio-typologies. In particular, some taxa (i.e. Isoperla, Hydropsychidae, Rhyacophilidae and Ephemerella) have showed higher adaptation levels than expected by the I.B.E. scores. This first contribution has underlined the importance of the disposal of basic elements on the autoecology of single indicative taxa for a critical approach on the composition of the reference communities and the ecotypes identification. © Asociación Española de Limnología, Madrid.

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