We investigated the effectiveness of surface colonization by the epiphytic marine bacterium Pseudoalteromonas tunicata firstly on a complex biofilm community on glass slides, and secondly, on the epiphytic community of Ulva australis. The effectiveness of P. tunicata was compared with the performance of Phaeobacter sp. 2.10, also a marine epiphytic isolate in the U. australis colonization experiments. Pseudoalteromonas tunicata cells were able to colonize the glass slide community at densities found naturally in the water column (9.7 × 104 cells mL−1). However, P. tunicata was a poor invader of the epiphytic community on U. australis at densities of 106 cells mL−1. At densities of 108 cells mL−1, P. tunicata again exerted little impact on the epiphytic community. Phaeobacter sp. 2.10 was also a poor invader at lower densities, but was able to invade and become dominant at densities of 108 cells mL−1. Differences in the ability of P. tunicata and Phaeobacter sp. 2.10 to invade natural communities may be due to differences in the antibacterial compounds produced by the two species. These experiments suggest that epiphytic communities may have protective effects compared with inanimate surfaces.