In general the anaerobic digestion leads to a reduction of the pathogenic microorganisms in the fermented substrat. The level of the reduction is determined by the reaction temperature and by the hydraulic retention time of the slurry in the digester. In 1986 investigations were carried out in the agricultural biogas plant in Vippachedelhausen. The main results are: The decrease of microorganisms during the agricultural biogasproduction is not so strength as described by other authors. A reduction of about one order of magnitude was measured. In the case of 10(7) to 10(9) microorganisms inside the start slurry the reduction is only of a small practical importance. This small reduction is caused by the short hydraulic retention time of about 12 days in connection with the principle of the stirred tank reaktor.