Samples from 95 papaya plants with symptoms of papaya bunchy top (PBT) were obtained with the aid of local cooperators from 12 countries throughout the American tropics. The samples were examined by polymerase chain reaction (PCR) for the presence of 16S rRNA genes of mycoplasma-like organisms, but none were found. No differences were detectable by PCR between PBT-affected papaya DNA samples and samples from healthy plants. Abnormal fluorescence was consistently observed by epi fluoresce nee microscopy in the region between the xylem and phloem in transverse sections of PBT-affected petioles when stained by 4',6-diamidino-2-phenylindole2HCl (DAPI) or acridine orange. Bacteria were found in the region by transmission electron microscopy and observed in expressed sap by light microscopy. The bacteria were associated with PBT-affected, but not with healthy, papaya plants. The bacteria measured 0.25-0.35 μηι in width and 0.8-1.6 Km in length and had a Gram-negative type cell envelope. All attempts to isolate the bacterium in axenic culture have failed.