Group I introns, which are widespread in nature, carry out RNA self-splicing. The secondary structure common to these introns was for the most part established a decade ago. Information about their higher order structure has been derived from a range of experimental approaches, comparative sequence analysis, and molecular modelling. This information now provides the basis for a new two-dimensional structural diagram that more accurately represents the domain organization and orientation of helices within the intron, the coaxial stacking of certain helices, and the proximity of key nucleotides in three-dimensional space. It is hoped that this format will facilitate the detailed comparison of group I intron structures.