Quaternary ammonium compounds (QACs) are cationic microbicidal substances which are important ingredients of disinfectants used for the disinfection of surfaces and medical instruments (Russel et al. 1992). The use of QACs as microbicidal ingredients increased during the last decade as substitute for aldehydes due to the health risk of aldehydes to humans. QACs are emitted via hospital effluents into municipal sewage (Kümmerer et al. 1997) and can disarrange biological waste water treatment. The prevalence of QACs resistant bacteria is elevated in effluents of sewage treatment plants receiving sewage containing QACs (Hingst et al. 1995). QACs are reported to be not readily biodegradable (De Wart 1986, Van Ginkel 1991, ECETOC 1993). Because of the adsorption of QACs contradictory results of biodegradability, elimination and toxic effects of these substances against microorganisms in different biodegradability tests were reported (Gerike et al. 1978, ECETOC 1993, Sanchez Leal et al. 1994). One reason for the diverging test results may be that up to now possible interactions between QACs and inoculum i.e. toxicity against the bacteria used and adaptation were not investigated. But such a closer