The gram-positive bacterium, Streptococcus iniae, causes infection in a wide range of farmed fish species and leads to massive production losses on fish farms. S. iniae also infects humans that have handled diseased fish. Vaccination of farm fish against S. iniae is widespread, with most vaccines comprising formalin killed bacterins. However, vaccination is not always successful, with vaccinated fish succumbing to reinfection by novel serotypes. In Australia, use of autogenous monovalent and polyvalent vaccines against S. iniae has led to rapid evolution of novel serotypes based on rapid mutation of key genes in the capsular operon. cpsG within the polysaccharide capsular operon has two indels that correlate with differing strain types and consequent vaccine escape. cpsG, encodes a UDP-glucose 4’-epimerase, a key enzyme in galactose metabolism that is involved in capsular polysaccharide (CPS) biosynthesis. To investigate what effect these mutations have on capsule biosynthesis, recombinant proteins of each variant were expressed and tested for enzyme function using a coupled spectrophotometric assay to determine whether the indel strains alter epimerase activity. Allelic replacement of cpsG was performed and buoyant density assays showed differing capsular production amongst the variants, possibly resulting from altered glucose/galactose ratio in the CPS. Further analysis of CPS from each cpsG variant by GC-MS is currently underway to determine whether this hypothesis is supported.