Two isoquinoline alkaloids, including the new compound 1, were isolated from the cytotoxic fractions of an aqueous extract of the marine sponge Haliclona sp. The structures of these compounds were established as 1-hydroxymethyl-7-methoxyisoquinolin-6-ol (1) and mimosamycin (2) by conventional spectroscopic methods and by comparison with related compounds. Mimosamycin (2) was the principal cytotoxin with an IC(50) of approximately 10 microg/mL against melanoma and ovarian human tumor cell lines.