Abstract: A portable toxicity tester for use in the field using bioluminescent organisms consisting of a light tight container for housing the necessary flash of light detecting apparatus and the necessary signal processing circuits, digital readout and control panel. A light tight chamber is provided with a removable stirrer assembly which is the stirrer motor housing as well as the light tight cover for the chamber. With the cover removed, a vial containing the necessary inoculated bioluminescence organisms is inserted into a holder in the chamber. Part of the stirrer assembly is a paddle attached to the motor shaft that extends to near the bottom of the vial. The top of the vial holder extends beyond the outer surface of the cover to insure a light tight seal.