The carboxyl- and amino-terminal ends of streptavidin are near the site of protein-protein contacts in two-dimensional streptavidin crystals. The role of these C- and N-terminal residues in determining the pH-dependent phase behavior of crystallization has been investigated with site-directed truncation mutants. Commercial streptavidin (consisting primarily of amino acids 14-136) and two recombinant streptavidin forms, spanning residues 13-136 and 13-139, have been crystallized at pH 4-7. The commercial 14-136 protein crystallizes in three distinct lattice symmetries, P1, P2, and C-222, respectively, depending on pH. The 13-136 mutant also crystallizes in three distinct lattices, but with a shifted pH profile that is attributed to the N-terminal residue. The presence of amino acids 137-139 inhibits the growth of crystals with P1 symmetry at low pH. In addition, we observe a solid-solid phase transition in situ from the P2 to the P1 crystal forms for the 13-136 recombinant protein at pH 5.2. We also demonstrate the ability of Brewster angle microscopy to distinguish between different crystal forms if protein monolayer densities are sufficiently different.