Abstract: A method and apparatus for testing for the presence of selected microorganisms can be used in the field or in the laboratory. A sample to be tested is mixed with a culture medium for promoting growth of the selected microorganisms. A test device of a semi-permeable material is inverted in the culture medium, and the test device is held to a vertical position. The microorganisms create a biofilm within the test device, and the test device subsequently retains gas generated by the microorganisms. The gas lowers the density of the test device, causing the test device to float. The floating is taken as an indication of the presence of the selected microorganisms; and, by measuring the time required for the test device to float, the size of the population of the selected microorganisms can be determined.