Moraxella catarrhalis is one of the major bacterial causes of otitis media, along with Streptococcus pneumoniae and non-typable Haemophilus influenzae. The virulence of S. pneumoniae and H. influenzae is influenced by extracellular nucleases (EndA and Nuc, respectively), that mediate biofilm formation (multicellular structures that enhance host colonisation and increase antibiotic resistance) and increase resistance to neutrophil killing, via destruction of extracellular traps. M. catarrhalis also has an extracellular nuclease, but it has not yet been characterised. We identified an open reading frame that encodes a predicted nuclease containing the DRGH motif that is seen in nucleases from the Nuc superfamily. To investigate this gene, termed nuc, we created knock out mutants by interrupting it with a kanamycin resistance cassette in two different strains of M. catarrhalis. Phenotypic analyses were then carried out to compare parental and mutant DNAse activity, growth, transformation efficacy, biofilm formation, antibiotic resistance. Our studies show that nuc is an extracellular factor that digests DNA and is needed for optimal transformation efficacy by M. catarrhalis. Deletion of the gene does not affect the planktonic growth of the cell, but M. catarrhalis Δnuc mutant strains are deficient in biofilm formation and have decreased antibiotic resistance. These data suggest that Nuc may be important for the virulence of M. catarrhalis in infections, as has been seen with nucleases in other otopathogens. To explore further roles in pathogenesis, Nuc is currently being investigated in neutrophil survival and cell adherence assays.