The crystal structure of ribosomal protein L30 from the extreme thermophilic bacterium Thermus thermophilus has been determined at 1. 9 A resolution. The crystals are trigonal and belong to space group P3(2)21, with unit-cell parameters a = b = 63.5, c = 77.8 A, alpha = beta = 90, gamma = 120 degrees and two molecules per asymmetric unit. The structure was solved by the molecular-replacement method with AMoRe and refined with X-PLOR to an R value of 20.3% and an R(free) of 25.3% in the resolution range 8-1.9 A. Detailed analyses of the structures of the two molecules in the asymmetric unit and comparison of T. thermophilus L30 structure with the structure of homologous L30 from Bacillus stearothermophilus reveal two flexible regions at opposite ends of the rather elongated molecule. Such flexibility could be important for the protein fitting in the ribosome. A comparison with B. stearothermophilus L30 shows a higher number of salt bridges and unbound positively charged residues and an increased accessible hydrophobic area on the surface of T. thermophilus L30. This could contribute to the stability of both the extreme thermophile protein and the ribosome as a whole.