Columnaris-causing bacteria (CCB; formally Flavobacterium columnare), are gram-negative , yellow-pigmented bacteria that are responsible for columnaris disease . As of 2022, CCB represent four distinct species: F. columnare, F. covae, F. davisii , and F. oreochromis, each having an association between species and virulence in different fish hosts. In the southeastern United States, columnaris disease causes significant losses in catfish aquaculture, and control methods rely heavily on antibiotics. The development and availability of an efficacious vaccine for the prevention and control of columnaris disease has been hampered, partially, due to a lack of understanding of the broad genetic diversity of CCB. Vaccine development has been re-initiated with the newfound knowledge of the genetic diversity of CCB. Multiple strains of F. covae , F. davisii , and F. oreochromis that are virulent in channel catfish (Ictalurus punctatus) or Nile tilapia (Oreochromis niloticus ) are currently being evaluated as candidates for a live-attenuated CCB vaccine. Mutant strains will be characterized and investigated for protective immune responses in channel catfish and Nile tilapia. Safety testing and delivery optimization (immersion or oral administration routes) will be performed on the most promising candidate. To date, six strains of virulent CCB have been attenuated. Mutant isolates of F. oreochromis and F. davisii showed a total loss of virulence in Nile tilapia (0% cumulative percent mortality; CPM), while wild-type isolates had >60% and >80% CPM, respectively. In two separate trials, an F. covae mutant showed significantly less CPM (10% and 30%) compared to the highest dose of the parent wild-type strain, which was 100% CPM in both trials. Results of ongoing vaccination trials with other mutant F. covae isolates will be discussed. The long-term goal of this project is to produce a viable vaccine to protect catfish and other essential aquaculture species in the southern region of the U.S. against columnaris disease.