Recombinant Scapharca homodimeric hemoglobin has been expressed at high levels from a synthetic gene in Escherichia coli. Addition of the heme precursor delta-aminolevulinic acid to the expression culture results in a considerable increase in the yield of soluble hemoglobin. The recombinant hemoglobin exhibits cooperative oxygen binding properties indistinguishable from native protein. Crystals of the recombinant protein, like those of native hemoglobin, diffract to high resolution which will allow functional studies of site-directed mutants to be correlated with detailed structural analyses.