Bacteria control major nutrient cycles and directly influence plant, animal and human health. However, we know relatively little about the forces shaping their large-scale ecological ranges, community assembly and stability over time. As for many living surfaces in the marine environment, the epiphytic bacterial biofilm communities on macro algae such as Macrocystis pyrifera, and the mechanisms that drive their assembly are poorly understood, apart from the magnitude and diversity of their composition. This biodiversity is not lacking for possible explanations, from colonization to dispersal, succession and persistence by competition. High throughput sequencing has contributed to a variety of potential mechanisms being proposed to characterize the assembly of these microbial communities. We used 454 parallel tag pyrosequencing of the 16s rna gene to better explore the diversity, stability and specificity of the microbial communities associated with the dominant kelp species in Monterey Bay. We also compared the kelp-associated communities with those present in the surrounding seawater to assess the influence of environmental conditions. Our results reveal patterns in the distribution of individual bacterial members at multiple levels of phylogenetic resolution within and around a M. pyrifera ecosystem suggesting the lack of a proposed stable core community of species associated with marine macro algae across space and time (Tujula et al., 2010). M. pyrifera harbors species-specific and temporally adapted epiphytic bacterial biofilms on their surfaces, and the most cosmopolitan taxa are also the most abundant in individual assemblages. Since several of the kelp-specific bacteria identified were highly similar to other bacteria known to either avoid subsequent colonization by eukaryotic larvae or to exhibit antibacterial activities, host-specific bacterial associations might play an important role for M. pyrifera. This study reports on the first published in-depth assessment of the diversity and phylogenetic profile of the bacterial communities associated with M. pyrifera.