Subtidal assemblages of epibiota on vertical and on horizontal surfaces of two natural reefs and two concrete breakwalls were sampled photographically during autumn and winter of 1998. Differences in the assemblages on the two types of substrata (natural reefs and concrete breakwalls) were detected between assemblages on horizontal surfaces, but not on vertical surfaces. The covers of several individual taxa (e.g. Herdmania momus, serpulid polychaetes, coralline encrusting algae) and number of sponge taxa showed clear differences between the two types of substrata. There were great differences between the assemblages on vertical and horizontal surfaces on each natural reef and artificial structure. Invertebrates consistently covered a larger area on vertical than on horizontal surfaces with sponges (as a group) and the ascidian Herdmania momus, the dominant invertebrates on these reefs, clearly showing this pattern. Nevertheless, this pattern was complex for sponges because several species covered a larger area on horizontal than on vertical surfaces and there was no difference in the number of taxa of sponges between the two orientations on natural reefs. Algae, contrary to the results of previous studies, did not show any consistent differences in their covers on vertical or on horizontal surfaces. The results of this study indicated that orientation may be of greater influence on the biological diversity of epibiota on subtidal reefs than whether reefs are natural or artificial.