COMPARISON OF CHROMATOPHOROTROPINS FROM THE HORSESHOE CRAB LIMULUS POLYPHEMUS, AND THE FIDDLER CRAB, UCA PUGILATOR 1. 1. Extracts of the central nervous system of the horseshoe crab, Limulus polyphemus, were assayed for chromatophorotropic activity on the fiddler crab, Uca pugilator. The extracts caused pigment dispersion in the melanophores and pigment concentration in the leucophores but had no effect on the erythrophores. 2. The ethanol-soluble and 95% methanol:chloroform (2:1)-soluble fractions of the central nervous system from the horseshoe crab evoked melanin-dispersing and white pigment-concentrating responses which had larger amplitudes and longer durations than did the responses caused by extracts prepared directly in saline. 3. Neither gel filtration nor acetone fractionation was effective in separating the melanin-dispersing activity from the white pigment-concentrating activity in extracts of the central nervous system of the horseshoe crab. These responses appear to be caused by either the same molecule or by different substances having similar elution patterns from Bio-Gel P-6. 4. Extracts of the eyestalk of the fiddler crab were fractionated on Bio-Gel P-6. The melanin-dispersing, white pigment-dispersing and red pigment-dispersing activities were eluted with an Rf of 0.57, the same value as that of the major peaks of melanin-dispersing and white pigment-concentrating activities from the central nervous system of the horseshoe crab. In contrast, the white pigment-concentrating and red pigment-concentrating activities of the fiddler crab separated from the pigment-dispersing activities, having been eluted later from the column of Bio-Gel P-6 with an Rf of 0.28. 5. The chromatophorotropic material in the central nervous system of the horseshoe crab is more closely related to the melanin-dispersing material of the fiddler crab than to the white pigment-concentrating hormone of the fiddler crab. 6. The chromatophorotropins in the central nervous system of the horseshoe crab and the radial nerves of the sea star differ from each other in their elution patterns from a column of Bio-Gel P-6 and in the chromatophore responses they evoke in the fiddler crab.