1. 1. Crystallizable RNA fragments, previously obtained by partial alkaline digestion of ribosomes, are a potentially important source of information about ribosome structure. The aim of the present work was to establish that fragment crystallization is a specific indication of well-defined structure in rRNA, rather than being an irrelevant artefact. 2. 2. Crystallizable fragments have now been produced by treatment of isolated rRNA. When alkali is used the fragments from yeast and rabbit reticulocyte rRNA can be made to crystallize, but those from Escherichia coli rRNA cannot. When yeast ribosomal nuclease is used, fragments from both yeast and E. coli rRNA are crystallizable. When T 1 ribonuclease is used on yeast rRNA the fragments are crystallizable, but when pancreatic ribonuclease is used they are not. RNA from both subunits of yeast ribosomes can yield crystallizable fragments. 3. 3. Fragments crystallize only when their molecular weights lie approximately in the range 9000 to 20 000 (the number of chains per particle remains to be determined). Compared with intact rRNA, crystallizable fragments obtained by treatment with ribosomal nuclease and T 1 ribonuclease contain less adenine and guanine, respectively. This suggests that the non-helical regions of rRNA are rich in purines; such a hypothesis would also explain the failure of pancreatic ribonuclease to yield crystallizable fragments. The results so far support the idea that crystallizable RNA fragments are derived from helical regions which are present in all types of rRNA and which are a fundamental unit of ribosome structure. The ultimate aim is to determine how these regions interact to build up the structure of the ribosome.