A novel Siphoviridae family Phage BS5, which infects Pseudoalteromonas atlantica, was isolated from the surface waters of the Yellow Sea. Morphological study by transmission electron microscopy revealed that the novel phage belongs to Siphoviridae. The complete genome sequence of PBS5 contained a linear, double-strand 39949-bp DNA molecule with a G + C content of 40.6% and 65 putative open reading frames. Twelve conserved domains were detected by BLASTP in NCBI, and of these the functions of 5 were known. The genome was grouped into four modules as follows: phage structure, phage packaging, DNA replication and regulation, and some additional functions.16 S rDNA sequence analysis was also applied to identify the host bacteria. After initial characterization of bacteriophage PBS5, it was found that the optimal pH was 7.0, the optimal temperature was 30 A degrees C, and the burst size was about 95 virions per cell. This information will provide an important benchmark for further research on the interaction between bacteriophages and their hosts.