The upper surfaces of sublittoral hard substrates in the Deer Island region of the Bay of Fundy, New Brunswick, support diverse, depth-zoned epibenthic communities of which ascidians form a minor part. Their population density was quantitatively studied from photo-transects taken between mean low water (MLW) and 30 m depth at four sites off the Deer Island coast and from 30–140 m depth along two photo-transects in Head Harbour Passage. All photo-analyses were aided by collections from transect survey sites, wharf pilings and salmon cage floats, to yield a total of 15 ascidian species encountered. Ascidians were found at all depths at the four shallow sites. Halocynthia pyriformis and Boltenia ovifera are most common at depths of less than 20 m, while Aplidium pallidum, Didemnum albidum and other species exhibit a marked increase in abundance below this depth. Cluster analysis of ascidians shows an association between B. echinata and B. ovifera , which may reflect resource partitioning, and between A. pallidum-D. albidum and Molgula sp.— A. stellatum , the ecological significance of which are as yet unknown. The community in Head Harbour Passage is animal-dominated and in its deeper sections often shows three-dimensional bottom relief from horse mussel shells. D. albidum , the commonest ascidian, shows a close association with Modiolus modiolus , to which it is normally attached, suggesting that mussel beds may minimize the possibility of dislodgement and even confer a feeding advantage on this ascidian.