A high frequency of dieback and mortality was found in Rhizophora mangle (red mangrove) in coastal mangrove forests of southwest Puerto Rico. Eight plots were established in a variety of mangrove environments to assess the extent and severity of the dieback and mortality. The imperfect fungus Cytospora rhizophorae was isolated consistently from stem dieback and associated canker tissues. No other pathogenic microorganisms were isolated. Pathogenicity was demonstrated in greenhouse and field inoculation experiments with C. rhizophorae using young R. mangle seedlings. In field studies of seedlings inoculated, mortality was 33 percent, and greenhouse mortality was as high as 50 percent. Cytospora rhizophorae was isolated consistently from diseased mangrove tissue; the fungus was grown in pure culture and then inoculated into healthy red mangroves, causing cankers. Koch's postulates were fulfilled by infecting R. mangle with C. rhizophorae, producing disease symptoms, and successfully re-isolating C. rhizophorae from the diseased tissue. This is the first documentation of the pathogenicity of C. rhizophorae associated with mortality in red mangrove. In the coastal mangrove forests of southwest Puerto Rico, C. rhizophorae may play an important role in the frequency of dieback and mortality in R. mangle, producing slow stem diameter growth and causing frequent stem and root wounds.