Phosphopentomutases (PPMs) interconvert D-ribose 5-phosphate and alpha-D-ribose 1-phosphate to link glucose and nucleotide metabolism. PPM from Bacillus cereus was overexpressed in Escherichia coli, purified to homogeneity and crystallized. Bacterial PPMs are predicted to contain a di-metal reaction center, but the catalytically relevant metal has not previously been identified. Sparse-matrix crystallization screening was performed in the presence or absence of 50 mM MnCl(2). This strategy resulted in the formation of two crystal forms from two chemically distinct conditions. The crystals that formed with 50 mM MnCl(2) were more easily manipulated and diffracted to higher resolution. These results suggest that even if the catalytically relevant metal is not known, the crystallization of putative metalloproteins may still benefit from supplementation of the crystallization screens with potential catalytic metals.