The present work was aimed at studying antagonistic interactions existing among cultivable bacteria associated with the Antarctic sponges Anoxycalyx joubini and Lissodendoryx nobilis. Overall, bacterial isolates were affiliated with the alpha- and gamma-Proteobacteria (17.3 and 65.3%, respectively), the CFB group of Bacteroidetes (10.7%) and the Actinobacteria (6.7%) by 16S rDNA sequencing. The two sponges harbored microorganisms belonging to different species/genera and previously retrieved from polar marine environments. Antagonistic interactions, assayed by the cross-streak method and statistically analyzed using the "network theory" approach, were checked among isolates associated with the same sponge as well as between isolates retrieved from the two sponge species ("cross-niche inhibition"). Results suggest that antagonism could play a significant role in shaping bacterial communities within sponge tissues. Data from this study confirm previous observations on the antibacterial activity of Antarctic microorganisms and represent a baseline for further investigation of both the ecological role and biotechnological exploitation of Antarctic sponge-associated bacteria.