Lactococcus lactis is the most widely utilised starter bacterial species in dairy fermentations. The L. lactis cell envelope contains polysaccharides, which, among other known functions, serve as bacteriophage receptors. Our previous studies have highlighted the structural diversity of these so-called cell wall polysaccharides (CWPSs) among L. lactis strains that could account for the narrow host range of most lactococcal bacteriophages. In the present work, we studied the CWPS of L. lactis strain UC509.9, an Irish dairy starter strain that is host to the temperate and well-characterized P335-type phage Tuc2009. The UC509.9 CWPS structure was analyzed by methylation, deacetylation/deamination, Smith degradation and 2D NMR spectroscopy. The CWPS consists of a linear backbone composed of a tetrasaccharide repeat unit, partially substituted with a branched phosphorylated oligosaccharide having a common trisaccharide and three non-stoichiometric substitutions.