Description: Interactions between epibiotic bacteria and organisms possibly play a central role in marine ecology. Despite its potential significance, this held has long time been neglected. For most aquatic taxa nothing is known about presence/absence of bacteria on their. surface, much less about specific associations or potential interactions between epibiotically associated micro- and macroorganisms. Bahamian and Pacific ascidians, most of them colonial, were screened for the presence, abundance and diversity of epibiotic bacteria and macroepibionts. Only one species, Polyclinum planum, occasionally carried macroepibionts. All ascidian species exhibited varying densities of epibiotic bacteria on their surfaces. Average epibacterial abundance as assessed by plate counts on the 29 species ranged from 60 to 1.2X10(7)/cm(2). Significant differences in bacterial abundances were observed between species, families and geographical regions. On the family level, Polyclinidae were the most densely colonized. Bahamian species exhibited less dense epibacterial communities than Pacific species, a difference that may partly be caused by the absence of the heavily fouled Polyclinidae from the Bahamian collection. Diversity of culturable strains, evaluated for the Bahamian species only, was uniformly high on most species. I did not find any evidence for specific associations (as reflected by dominance of single strains) between culturable bacteria and ascidian species. Contrarily, direct observation by epifluorescence revealed the presence of an apparently dominant photosynthetic symbiont on several didemnid species. The presence of this symbiont correlated negatively with abundance and diversity of culturable epibionts. This negative correlation could reflect properties of the host's surface which selectively favor proliferation of the symbiont or antagonistic interactions between the symbionts and other potential bacterial colonizers.