A new hexahydrophenanthrene sulfate was identified from water conditioned with sea lamprey larvae ( Petromyzon marinus ) and named petromyzonin. Its structure was unequivocally elucidated on the basis of spectroscopic analyses including comparison with spectra of known compounds. The absolute configuration was determined by electronic circular dichroism. Petromyzonin may function as a chemical signal, as it elicited responses in electro-olfactogram recording with a dynamic concentration-response relationship and a detection threshold of 10(-11) M.