Abstract: The invention relates to an apparatus for registering the presence of bacteria, particularly under field conditions. The apparatus includes a plate (1), in which there is formed a luminescence chamber (2) which is defined at its opening (3) by a support lattice (5) carrying a filter (4), the lattice opening out into and being carrying sealingly against the plate (1) with the aid of a liquid-sealing and opaque collection vessel (6). The plate (1) includes a transparent portion (7) which is situated above or opposite the opening (3) of the chamber (2). There is a means (8) above the portion (7) for registering a light flux caused by luminescence, said means (8) being accommodated in an opaque housing (13) sealing against the plate (1), and there is an aperture plate (9) displaceably arranged above the portion (7) between the plate (1) and the means (8) to prevent unintentional exposure of the means.