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Our study attempted a new approach to biological assessment in Germany that would comply with the requirements of the European Union Water Framework Directive. We developed a multimetric index for use throughout Germany, based on the macrozoobenthos and devised along the guidelines formulated by Karr & Chu (1999). The index contains twelve measures, chosen for their discriminatory power in assessing general impairment as well as specific stressors (e.g., impoundments, acidification, organic or chemical pollution), the inclusion of all meaningful ecological metric categories, interrelationship as low as possible and the feasibility of using them in all geographic stream classes of Germany. Due to the availability of data over time the method was developed in three steps. In step 1 we studied the stressor specific response of biological attributes to additional stress by comparing each of 162 sites impaired by a known factor with a nearby site, which was morphologically comparable, but impaired to a lesser extent. The resulting list of 17 candidate metrics for a multimetric index was validated in step 2 using a set of model data from our own investigations. This step included data from not or minimally disturbed reference sites and the design of a five-class-scale of human impact to permit the use of dose-response curves, but no differentiation of stream types. The multimetric index IBI 12 was developed using dose-response curves, correlation coefficients and graphical analysis. The index was improved in the third step, calculations for this based on a dataset containing about 4000 macrozoobentos samples from over 900 streams and rivers in Germany, collected by water management authorities and researchers. The final version of the method uses type specific reference conditions, but the same set of 12 metrics for all stream types. This new IBI 12 is suitable to fulfil the requirements of the EU Water Framework Directive. It correlates with a Spearman's R of 0.76 with the general state of impairment for every stream type, and a Spearman's R between 0.6 and 0.87 with the most important specific stressors like sewage or morphological degradation. It proved to correlate higher with those stressors than any single index as yet used in Germany. The stressor specific analysis also substantiated that no single metric was really stressor specific with the exception of an acidity index. Even the well established Saprobic indices responded quite well to other stressors besides organic pollution. The results suggest that a separate assessment of specific stressors might be only possible by analysing the reaction pattern of an array of metrics. Further improvement of the index could be achieved by advances in stream typology and the reference conditions, as well as by a higher degree of standardisation to decrease the scatter caused by variations in sampling season, methodology and determination level. [ABSTRACT FROM AUTHOR]