This fluorescent substrate analog of phytic acid was shown to allow direct measurement of the phosphateester bond-cleavage reaction for determination of phytase activity.The TET TInsP5 molecular probe should prove useful for assessing the nutritive status of P in surface water ecosystems. The advent of a fluorescent substrate analog of phytic acid affords environmental scientists the means to unambiguously quantify an extremely small amount of phytase-generated product(s), enabling the measurement of phytase activity, over a reasonably short time duration, in an environmental sample containing low concentrations of enzyme.