ABSTRACT-Competition experiments were performed using precolonized leaves or leaf disks of red mangrove Rhizophora mangle with: (1) disks containing pure cultures of single species of marine true fungi or species of Halophytophthora (the principal genus of marine oomycotes); and (2) leaves bearing bacterial films. Preoccupied leaves were exposed to natural microflorae in mangrove creeks at 2 Cays in the Bahaina Islands, or placed in laboratory seawater enclosures wherein pairs of halophytophthoras were given equivalent opportunity to occupy fresh leaf material. The ubiquitous coastal-marme oomycote H. vesicula was found to be an able competitor versus true fungi and versus other halophytophthoras. Against other halophytophthoras, this was true for both primary and secondary resource capture. The one exception among the fungi was a species (Dendryphiella salina) common in decaying drift material in high-intertidal zones. H. spinosa was a weak competitor with true fungi and with H. vesicula, though it was not displaced by H. vesicula, and H. spinosa could depress the frequency of H. vesicula occupation when H. spinosa was well established. H. bahamensis did not routinely form sporangia, preventing identification and firm conclusions regarding compet~tiveness, other than that it could not block H. vesicula, but could block H. spinosa from entering its occupied arenas. When bacterial films were present on leaves prior to access by halophytophthoras, the occupation frequency of halophytophthoras was sharply depressed (by about 70 to 90% with 48 h bacterial films), including for H. vesicula, implying that in some types or parts of mangrove systems, submerged-leaf decomposition may sustain low levels of participation by halophytophthoras.