1. An apparatus for determining the concentration of microorganisms in a water specimen, said apparatus comprising a rigid body having a plurality of first wells and a plurality of second wells therein, the first wells being of equal volume and the second wells being of equal volume with the volume of the second wells being different from that of the first wells, the rigid body also having an inlet which opens outwardly from the body and forms an entry into which the water specimen may be introduced into the body, the rigid body further having filling channels leading up to and opening into the first and second wells, there being a separate filling channel for each first well and for each second well with each filling channel communicating with the inlet such that a water specimen introduced into the inlet passes into the filling channels and then into the wells, the upstream end of each filling channel being in communication with the inlet independently of the other filling channels so that the water specimen will flow into each well without passing through another well; means closing the ends of the wells and the sides of the filling channels for isolating the interiors of the wells and filling channels from the surrounding atmosphere except through the inlet, said means being transparent; and a culture medium in the wells.