The crystal structure of ribosomal protein L1 from the archaeon Methanococcus thermolithotrophicus has been determined at 2.7 A resolution. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 67.0, b = 70.1, c = 106.3 A and two molecules per asymmetric unit. The structure was solved by the molecular-replacement method with AMoRe and refined with CNS to an R value of 18.9% and an R(free) of 25.4% in the resolution range 30-2.7 A. Comparison of this structure with those obtained previously for two L1 proteins from other sources (the bacterium Thermus thermophilus and the archaeon M. jannaschii) as well as detailed analysis of intermolecular contacts in the corresponding L1 crystals reveal structural invariants on the molecular surface which are probably important for binding the 23S ribosomal RNA and protein function within the ribosome.