A marine sponge Petrosia similis afforded two compounds which belongs to bis-quinolizidine alkaloids namely, petrosin (1) and petrosin-A (2), respectively. Petrosin (1) and petrosin-A (2) showed anti-HIV inhibition with IC(50) values of 41.3 and 52.9 microm respectively. MAGI cell assays indicated that the compounds inhibited early steps of HIV replication. In extracellular HIV-1 Reverse Transcriptase inhibition assay petrosin and petrosin-A inhibited HIV-1 RT at 10.6 and 14.8 microm. This is the first report of petroisns with anti-HIV activity.