Three tricyclic guanidine alkaloids, including 1, 8a;8b, 3a-didehydro-8b-hydroxyptilocaulin (1), 1, 8a;8b, 3a-didehydro-8a-hydroxyptilocaulin(2) and mirabilin B (3), were identified from the marine sponge Monanchora unguifera.1, 8a;8b, 3a-Didehydro-8a-hydroxyptilocaulin(2) is a new stereoisomer of 1, the structure of which was elucidated by spectroscopic analysis, comparison of its spectral data with those of 1, and confirmed by X-ray analysis.Compounds 1 and 2 co-crystallized in an unusual perfect order and packed around an approximate inversion center. A mixture of 1 and 2 is active against the malaria parasite Plasmodium falciparum with an IC50 value of 3.8μg/mL while mirabilin B (3) exhibited antifungal activity against Cryptococcus neoformans with an IC50 value of 7.0μg/mL and antiprotozoal activity against Leishmania donovani with an IC50 value of 17μg/mL.