This review discusses the impact that molecular studies of the genic and spacer regions of the ribosomal cistron have had on the systematics and classification of the florideophyte red algae. Each ribosomal cistron consists of a non- transcribed spacer region (NTS), an external transcribed spacer region (ETS), the small-subunit (SSU) rRNA gene, internal transcribed spacer (ITS 1) region 1, the 5.8S rRNA gene, internal transcribed spacer (ITS 2) region 2, and the large-subunit (LSU) rRNA gene. To date, molecular investigations on the Rhodophyta that utilize the ribosomal cistron have been based largely on the SSU and, to a lesser extent, the ITS regions. We then outline the methods and procedures we use to obtain sequence data from the various regions of the ribosomal cistron. A discussion of recent advances in florideophyte systematics within the four florideophyte lineages outlined by Saunders & Kraft (1997) is presented. We then close the review with an introduction to, and discussion of the potential utility of sequences of the LSU for resolving relationships within the Rhodophyta.
