A new compound, N-(4-guanidinobutyl)-2-(4-hydroxyphenyl)-2-oxo-acetamide (1) was isolated from the aqueous extract of the hydroid Campanularia sp. Its structure was elucidated using NMR spectroscopic techniques and mass spectrometric analysis. The most stable conformation was determined using molecular modeling and the results of a NOESY experiment. Although compound 1 shows structural similarities to some highly potent histone deacetylase inhibitors (HDACi), e.g., suberoylanilide hydroxamic acid (SAHA) (2) and trichostatin A (TSA) (3), it does not inhibit the growth of ARP-1 cells at 100 microM concentration, a significant indication that it has no inhibitory activity to HDACs.