Abstract Background Spontaneous Bacillus anthracis mutants resistant to infection by phage AP50c (AP50^R) exhibit a mucoid colony phenotype and secrete an extracellular matrix. Methods Here we utilized a Roche/454-based whole genome sequencing approach to identify mutations that are candidates for conferring AP50c phage resistance, followed by genetic deletion and complementation studies to validate the whole genome sequence data and demonstrate that the implicated gene is necessary for AP50c phage infection. Results Using whole genome sequence data, we mapped the relevant mutations in six AP50^R strains to csaB. Eleven additional spontaneous mutants, isolated in two different genetic backgrounds, were screened by PCR followed by Sanger sequencing of the csaB gene. In each spontaneous mutant, we found either a non-synonymous substitution, a nonsense mutation, or a frame-shift mutation caused by single nucleotide polymorphisms or a 5 base pair insertion in csaB. All together, 5 and 12 of the 17 spontaneous mutations are predicted to yield altered full length and truncated CsaB proteins respectively. As expected from these results, a targeted deletion or frame-shift mutations introduced into csaB in a different genetic background, in a strain not exposed to AP50c, resulted in a phage resistant phenotype. Also, substitution of a highly conserved histidine residue with an alanine residue (H270A) in CsaB resulted in phage resistance, suggesting that a functional CsaB is necessary for phage sensitivity. Conversely, introduction of the wild type allele of csaB in cis into the csaB deletion mutant by homologous recombination or supplying the wild type CsaB protein in trans from a plasmid restored phage sensitivity. The csaB mutants accumulated cell wall material and appeared to have a defective S-layer, whereas these phenotypes were reverted in the complemented strains. Conclusions Taken together, these data suggest an essential role for csaB in AP50c phage infection, most likely in phage adsorption. (The whole genome sequences generated from this study have been submitted to GenBank under SRA project ID: SRA023659.1 and sample IDs: AP50 R1: SRS113675.1, AP50 R2: SRS113676.1, AP50 R3: SRS113728.1, AP50 R4: SRS113733.1, AP50 R6: SRS113734.1, JB220 Parent: SRS150209.1, JB220 Mutant: SRS150211.1).