One of the current problems in the field of coral disease research is that of tracking coral pathogens in the natural environment. A promising method to do this is by use of pathogen-specific molecular probes. However, this approach has been little used to date. We constructed, and validated in the laboratory, a fluorochrome-labeled molecular probe specific to Aurantimonas coralicida, the bacterial pathogen of the Caribbean coral disease white plague type II (WPIl). We then used the probe to test field samples of diseased coral tissue for the presence of this pathogen. Probe design was based on a unique subset (25 nucleotides) of the complete l6S rRNA gene sequence derived from a pure culture of the pathogen. The pathogen-specific probe was labeled with the fluorochrome GreenStar* FITC (fluorescein isothiocyanate, GeneDetect Ltd, New Zealand). As a control, we used the universal eubacterial probe EUB 338, labeled with a different fluorochrome (TRITC, tetra-methylrhodamine isothiocyanate). Both probes were applied to laboratory samples of pure cultures of bacteria, and field samples collected from the surface of the disease line of corals exhibiting signs of white plague (types I and II), healthy controls, and corals with an uncharacterized disease ("patchy necrosis"). All samples were analyzed using fluorescence in situ hybridization (FISH). We have determined that the probe is specific to our laboratory culture of the coral pathogen, and does not react with other bacterial species (the eubacterial probe does). The WPII pathogen was detected in association with diseased coral samples collected from coral colonies on reefs of the Bahamas (n= 9 samples) exhibiting signs of both WPI and WPII. Diseased (and healthy) tissue samples (n- 4) from corals exhibiting signs of "patchy necrosis" were also assayed. In this case the results were negative, indicating that the same pathogen is not involved in the two diseases. Incorporation and use of pathogen-specific probes can significantly expand our knowledge of the etiology of coral diseases.