The X-ray crystal structure of the fluoride derivative of Aplysia limacina ferric myoglobin has been solved and refined at 2.0 A resolution; the crystallographic R-factor is 13.6%. The fluoride ion binds to the sixth co-ordination position of the heme iron, 2.2 A from the metal. Binding of the negatively charged ligand on the distal side of the heme pocket of this myoglobin, which lacks the distal His, is associated with a network of hydrogen bonds that includes the fluoride ion, the residue Arg66 (E10), the heme propionate III, three ordered water molecules and backbone or side-chain atoms from the CD region. A comparison of fluoride and oxygen dissociation rate constants of A. limacina myoglobin, sperm whale (Physeter catodon) myoglobin and Glycera dibranchiata monomeric hemoglobin, suggests that the conformational readjustment of Arg66 (E10) in A. limacina myoglobin may represent the molecular basis for ligand stabilization, in the absence of a hydrogen-bond donor residue at the distal E7 position.