Millions of people worldwide are exposed to toxic levels of arsenic through contaminated drinking water. In last year’s project, we began the development of an E. coli-based arsenic biosensor using an arsenic-induced promoter from the Registry (K190015). Like the Groningen Team, we found that this promoter is “leaky,” having a constant low level of activity. We explored ways of reducing background activity to improve the sensitivity of our device. We have built and characterized biosensors using K190015 and a number of chemiluminescence generators from Vibrio fisheri and two species of fireflies. In order to standardize our biosensor we have designed and built a simple luminometer. Paired with our device, this can be used to inexpensively and rapidly detect arsenic in water, even by people in underdeveloped parts of the world.