The adhesin fimbriae-associated protein 1 (Fap1) is a surface protein of Streptococcus parasanguinis FW213 and plays a major role in the formation of dental plaque in humans. Increased adherence is highly correlated to a reduction in pH and acid activation has been mapped to a subdomain: Fap1-NR(α). Here, Fap1-NR(α) has been crystallized at pH 5.0 and diffraction data have been collected to 3.0 Å resolution. The crystals belonged to space group P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 122.0, c = 117.8 Å. It was not possible to conclusively determine the number of molecules in the asymmetric unit and heavy-atom derivatives are now being prepared.