Due to the frequent detection of antibiotics in the aquatic environment it has been investigated, whether a selection pressure can be stimulated by these substances in surface water within the microbial flora. The fate of two antibiotics, ciprofloxacin and ceftazidim, was investigated on the base of the closed bottle test, as a simple model for surface water, by means of HPLC in two concentrations respectively. Their effects on the inoculum were investigated by classic microbiological procedures. In high concentration ciprofloxacin proved as persistent and caused growth inhibition as well as strong shifts in the germ spectrum while it remained unobtrusive in low concentration within the scope of the test selection. Ceftazidim was eliminated approximately by half within 28 days, at which a large part was caused by abiotic elimination. The effects on the inoculum remained restricted in both concentrations to a test internal selection, which had to be attributed to the experimental conditions. A well-founded assumption exists, that the actually available average concentrations of ciprofloxacin and ceftazidim in surface water will be clearly below the low test concentrations, which caused no influence. Under this condition it seems to be improbable, that selective effects on bacterial communities are caused by these substances in surface waters. However, some methodic restrictions have to be taken into account.