1. 1. The inability of Porichthys from Puget Sound to luminesce is correlated with a luciferin deficiency in all developmental stages. 2. 2. Luminescence capability can be induced in non-luminous Porichthys by intraperitoneal injection or forced feeding with luciferin from either Cypridina or luminous Porichthys. 3. 3. The intensity of induced luminescence is dependent upon the dosage of unoxidized luciferin, whereas luciferin oxidation products and three analogues of Cypridina luciferin are ineffective. 4. 4. Extremely low luciferin doses are effective, and, once developed, luminescence capability is persistent. 5. 5. The mechanism of the induction process, whether involving incorporation or de novo synthesis of luciferin, remains unresolved.