An alignment of 75 phylogenetically diverse large subunit ribosomal RNA sequences was created and searched for secondary and tertiary structure elements by computer. The search revealed four unknown secondary structural pairings, two internal loop closings, and five short-range tertiary interactions--two of which were pairings of an unusual type. One brings a loop together with two other loops previously known to be paired, and one involves a nucleotide within a presumed tetraloop. The latter interaction constrains the RNA structure near the ribosomal E-site, where two base pairs previously suggested to be in parallel orientation are now proven. No clear phylogenetic evidence for direct base pairing between the large and small subunit rRNA was found.