The sequence of the 1173 nt 12S kinetoplast ribosomal RNA from Leishmania tarentolae was determined from the maxicircle DNA sequence, and the 5' and 3' ends localized by primer runoff and S1 nuclease protection experiments. The gene was shown to be free of introns by S1 nuclease analysis. A partial secondary structure model of the 12S RNA molecule is presented which is equivalent in certain respects to the corresponding portions of the Escherichia coli 23S ribosomal RNA model. Domain II of the E. coli model is completely missing in the kinetoplast model with the exception of several phylogenetically conserved stems and one loop. There is a striking conservation of the functionally important peptidyl-transferase region except for the deletion of a few stems and loops. The 12S RNA is the smallest large subunit ribosomal RNA described to date.