The recently developed electron microscopic technique of electron spectroscopic imaging has been used to map the distribution of phosphorus, and therefore of RNA, in situ in the ribosomal subunits of E. coli. The results indicate that the RNA moiety of both subunits is concentrated toward the centre of the particle somewhat more than is the total mass, but reaches the outer surface at several places. The micrographs also reveal certain distinctive features in the shape of the RNA component that may be related to the overall shape of the ribosome. The method yielded a reasonably accurate estimate of the phosphorus content of the 30 S ribosome.