The biosynthesis of poly(l-alanylglycine) (poly(AG)) was performed in high cell density cultures of recombinant Escherichia coli. The purity of the material was determined by amino acid analysis, elemental analysis, and 1H NMR spectroscopy. Fed batch fermentation increased the yield of recombinant protein from levels of tens of milligrams per liter (typical of batch fermentation in rich media) to hundreds of milligrams per liter. Poly(AG) comprising 64 diads [(AG)64] was recrystallized from dichloroacetic acid solutions in the form of texture-oriented chain-folded lamellae with a lamellar stack periodicity of 3.2 nm. The crystal structure within the lamellar core is similar in general, but different in detail, to the antiparallel β-sheet structure previously reported for oriented films of poly(AG) and fibers of Bombyx mori silk fibroin (silk II). The structure consists of polar antiparallel (ap) β-sheets, with repetitive folding through γ-turns every eighth amino acid (including the fold), stacking with l...