We report here on new 6-hydroxy-l,2,3,4-tetrahydropyrrolo[l,2-a]pyrimidinium alkaloids, belonging to the phloeodictyne family, isolated from the haplosclerid sponge Oceanapia [= Phloeodictyon] fistulosa (Browerbank, 1873) from New Caledonian shallow waters. Online LC-ESI-MS analysis, coupled to tandem fragmentation experiments on the crude alkaloid mixture, allowed us to clarify their flat structures, including structural isomers. At least 25 different components, of which 17 are new members with variable terminus and length chains, were characterised, besides less abundant analogues bearing a thioethylguanidine side chain. Crude mixtures and HPLC enriched fractions proved active against chloroquine-resistant Plasmodiumfalciparum, with IC50 values ranging from 0.6 to 6 μM, while cytotoxicity against human A-549 cell line was low. This makes these alkaloids a good prospect as leads for novel antimalarial agents.