In this paper, some important problems related to taxonomic resolution in water quality assessment by means of macroinvertebrates are discussed. Most quality indices based on macroinvertebrates only require identification up to genus or family level. Although this can be seen as a practical trade-off between taxonomic precision and time constraints and financial resources, it can result in biased assessment scores for certain stream types. An additional difficulty of identification levels other than species is caused by possible changes in taxonomy over time. A given genus may indeed have been split up into two or more genera or a species could be assigned to a different genus. These changes may alter biotic index values calculated over time, due to a change in number of taxa or replacement of one taxon by another one having a different tolerance class. An additional problem is caused by the invasion of exotic species. The genus Corbicula for instance is currently invading Belgian watercourses in increasing numbers. Since no Belgian Biotic Index (BBI) tolerance class is defined for Corbicula, this may cause inconsistencies in index calcu-lations as well. In order to eliminate these, a semi-fixed taxa list, including a tolerance class for each taxon, for BBI calculation is proposed
