Explosives are one of many hazardous waste problems of concern to the Department of Defense. Defective storage facilities or byproducts of weapons manufacture have led to contamination of soil and water with explosives. Most explosives are toxic, thus posing an ecological and human health hazard. The ability to do on-site or down-stream detection of explosives will be invaluable for site characterization and remediation by saving both time and money. The evanescent wave fiber optic biosensor that was developed at NRL has been modified for the detection of trinitrotoluene (TNT), by developing a competitive immunoassay on the surface of an optical probe. A fluorescently labelled analog of TNT, trinitrobenzenesulfonic acid (TNB), was used as the competitor. Enzyme-linked immunosorbent assays were performed to determine the best fluorescently labeled competitor available to be able to achieve high sensitivity in the fiber optic assay. For the competition assay, 7.5 ng/ml Cyanine 5-ethylenediamine-labelled TNB (Cy5-EDA-TNB) was exposed to an antibody-coated optical fiber generating specific signal above background that corresponds to the 100% or reference signal. Inhibition of this signal was observed in the presence of TNT with the percent inhibition proportional to the TNT concentration in the sample. Detection sensitivities in aqueous solutions containing 10 ng/ml TNT (8 ppb) have been achieved using this system.