The structure of a light-adapting hormone of the shrimp, Pandalus borealis, has been determined. The hormone, which had been isolated from Pandalus eyestalks and which adapts the shrimp to brighter light conditions by causing the pigment in the distal retinal pigment cells of the eye to move into a more proximal position, is the peptide: Asn-Ser-Gly-Met-Ile-Asn-Ser-Ile-Leu-Gly-Ile-Pro-Arg-Val-Met-Thr-Glu-Ala-NH2. The structure was obtained by sequence analysis by the dansyl-Edman method of the intact hormone and of isolated tryptic and thermolytic peptides.