Seven new guanidine alkaloids (1−7) together with the known batzelladines A, F, H, and L, ptilomycalin A, and fromiamycalin were isolated from the Caribbean marine sponges Monanchora arbuscula and Clathria calla. Molecular structures were assigned on the basis of detailed analysis of 1D and 2D NMR spectra and mass spectrometry data, and bioactivities of the alkaloids were evaluated against human cancer cell lines and malaria protozoa.