Previously, we isolated the M18 scFv, which is an affinity matured antibody against the anthrax toxin PA, and observed that its single chain antibody (scAb) form (M18 scAb) exhibited superior stability compared to the scFv. Here, we report high cell density cultivations for preparative scale production of M18 scAb in a 3.5 L fermenter. Briefly, a pH-stat feeding strategy was employed in fed-batch cultivation, and four different cell densities (OD(600) of 40, 80, 120, and 150) were examined for the induction of scAb gene expression. Among the four cell densities investigated, lower cell densities (OD(600) of 40) showed higher post-induction cell growth and production yields (665 mg/L of scAb). Even though lower solubility (51%) of scAb was achieved at lower cell density (OD(600) of 40), monomeric scAb could be purified with high purity (> 95%) using simple purification procedures. The purified scAb from high cell density cultures showed biological activity equivalent to that of scAb purified from shake flask cultivation.