1. The ctenidia of Phacoides pectinatus contain hemoglobin. 2. The hemoglobin appears to be located in the inner tissue of the ctenidia; it does not appear to be associated with the ciliated cells of the surface of the ctenidia. 3. In the cells in which it occurs, the presumptive hemoglobin is highly localized in the form of granular masses, of diameter about 5-7 µ, with the individual granules having diameters ranging from about 0.6 to 1.3 µ. 4. Evidence is presented that the hemoglobin has a sedimentation constant s20,w of 2.2 x 10-13 seconds, but this value needs confirmation. 5. The oxygen dissociation curve of the hemoglobin exhibits no evidence of heme-heme interaction. The pigment has a p50 of 0.19 mm. Hg at 25° C. in pH 7.4 0.2 M phosphate buffer in an impure preparation; this value is in accord with the ecology of the species.