Within the presented work, the suitability of aquatic fungi for the decolourisation of textile dyes was shown. By means of a two step screening, three promising strains were chosen. These three strains have proven affiliations with ascomycete teleomorphs. The decolourisation performances of these strains were essentially comparable to those of simultaneously tested white rot fungal strains, which were included as references. The culture conditions were stepwise altered to mimic real textile dyeing effluents during decolourisation experiments in shaken flasks. Strain Phoma sp. UHH 5-1-03 was able to reduce the colour and the chemical oxygen demand of solutions under model waste water conditions. A bioreactor system was optimised for this strain and successfully applied for the treatment of model waste waters up to the 10-L-scale. The involvement of a laccase produced by Phoma sp. UHH 5-1-03 in the biotransformation of dyes was demonstrated. The laccase was characterised biochemically and on the genetic level. It possesses unusual properties, which render this enzyme interesting for potential biotechnological applications. A statistical experimental design was used to optimise the production of the investigated laccase. The laccase titers reached are amongst the highest ones described for ascomycetes so far.