1. 1. Myoglobin was isolated from the red skeletal muscle of the dolphin fish Coryphaena hippurus (L.). The myoglobin was freed from minor components by chromatography on DEAE-Sephadex. 2. 2. The myoglobin migrated as a single component in polyacrylamide gel electrophoresis. Sedimentation equilibrium indicated a homogeneous preparation. The molecular weight was determined by sedimentation equilibrium and by detergent-gel electrophoresis. 3. 3. Absorption maxima and molar absorption coefficients were determined for the ferrimyoglobin. The pK′ for the acid dissociation of the haem-bound water molecule in the ferryimyoglobin was shown to be 8·89. 4. 4. The amino acid composition was determined. The myoglobin showed a low histidine and a high aspartic acid content.