An apparatus for continuously, periodically monitoring water for microbial germs, having a reactor (5) and at least one reagent feed line (18a..d) to the reactor and at least one metering device (19a..d) for reagents as well as at least one measuring device (16) for detecting the presence of microorganisms, wherein the reactor (5) has a reactor chamber (11) and a filtrate chamber (13) which is separated from the latter by a filter (12), a water feed line (1) and a reagent feed line (18a...d) open into the reactor chamber and at least one water discharge line leads from the filtrate chamber, and a controller (10) is set up to use pump/valve means (3, 4, 6, 7) to lead a predefinable quantity of water into the reactor chamber and through the filter and to introduce a predefinable quantity of reagent into the reactor chamber.