The intron content of plant organellar genes is a useful marker in molecular systematics and evolution. We have tested representatives of a wide range of monocotyledonous plant families for the presence of an intron (cox2 intron 1) in one of the most conservative mitochondrial genes, the cox2 locus. Almost all species analyzed were found to harbor a group II intron at a phylogenetically conserved position. The only exceptions were members of a single monocot family, the Ruscaceae: representatives of all genera in this family were found to lack cox2 intron 1, but instead harbor an intron in the 3' portion of the cox2 coding region (cox2 intron 2). The presence of cox2 intron 1 in families of monocotyledonous plants that are closely related to the Ruscaceae suggests that loss of the intron is specific to this family and may have accompanied the evolutionary appearance of the Ruscaceae. Interestingly, sequences that are highly homologous to cox2 intron 2 are found in a nuclear intron in a lineage of monocotyledonous plants, suggesting that the originally mitochondrial group II intron sequence was transferred to the nuclear genome and reused there to build a spliceosomal intron.