The distribution of ribosomal protein binding sites on the 16S ribosomal RNA molecule has been analyzed by limited ribonuclease hydrolysis of RNA-protein complexes, as well as by the interaction of individual proteins with RNA fragments purified from partial enzymatic digests. Of the six 30S subunit proteins known to interact directly with 16S RNA, proteins S4, S8, S15, S20, and, probably, S13 bind within a fragment produced by T 1 RNase (12S RNA) that comprises some 900 nucleotides and covers almost the entire 5′-terminal half of the 16S molecule. A fragment of 500-600 nucleotides (8S RNA) that is contiguous with 12S RNA and arises from the 3′-terminal portion of the 16S molecule is believed to contain the binding site for protein S7. Protein S15 interacts specifically with a sequence of about 135 nucleotides (4S RNA) that derives from 12S RNA after more extensive hydrolysis. Protein S4, but none of the other ribosomal proteins, binds to a 500-nucleotide fragment (9S RNA), generated by pancreatic RNase, that lies at the 5′-terminus of 16S RNA and is completely overlapped by the 12S fragment. A preliminary map of the binding sites is presented.