3-Deoxy-D-manno-octulosonate-8-phosphate (KDOP) synthase catalyzes the production of KDOP from phosphoenolpyruvate (PEP) and arabinose-5-phosphate (A5P). In gram-negative bacteria KDOP is subsequently dephosphorylated, cytidylylated, and linked to lipid A and is required for lipid A incorporation into the outer membrane (Raetz, Annu. Rev. Biochem. 59:129-170, 1990). We have crystallized two forms of KDOP synthase belonging to space groups I23 or I2(1)3, one with a = b = c = 118.0 A and the other with a = b = c = 233 A.