Residue Phe97, which is thought to play a central role in the cooperative functioning of Scapharca dimeric hemoglobin, has been mutated to leucine to test its proposed role in mediating cooperative oxygen binding. This results in an 8-fold increase in oxygen affinity and a marked decrease in cooperativity. Kinetic measurements of ligand binding to the Leu97 mutant suggest an altered unliganded (deoxy) state, which has been confirmed by high resolution crystal structures in the unliganded and carbon monoxide-liganded states. Analysis of the structures at allosteric end points reveals them to be remarkably similar to the corresponding wild-type structures, with differences confined to the disposition of residue 97 side chain, F-helix geometry, and the interface water structure. Increased oxygen affinity results from the absence of the Phe97 side chain, whose tight packing in the heme pocket of the deoxy state normally restricts the heme from assuming a high affinity conformation. The absence of the Phe97 side chain is also associated with diminished cooperativity, since Leu97 packs in the heme pocket in both states. Residual cooperativity appears to be coupled with observed structural transitions and suggests that parallel pathways for communication exist in Scapharca dimeric hemoglobin.