Understanding the mechanisms by which ribozymes catalyse chemical reactions requires a detailed knowledge of their structure. The secondary structure of the group I introns has been confirmed by comparison of over 70 published sequences, by chemical protection studies, and by genetic experiments involving compensatory mutations. Phylogenetic data can also be used to identify tertiary interactions in RNA molecules. This was first done by Levitt, who predicted tertiary interactions in transfer RNA, which were subsequently confirmed by X-ray crystallography. More recently, sequence comparison data have been used to predict tertiary interactions in ribosomal RNA. We have searched a complete alignment of the core regions of group I introns for evolutionary covariations that could not be ascribed to classical Watson-Crick or wobble base pairings. Here we describe two examples of phylogenetic covariation that are most simply explained by postulating hydrogen-bonded base-triples similar to those found in tRNA. Genetic experiments with the Tetrahymena and sunY introns confirm the importance of these interactions for the structure of the ribozyme.