Phylogenetic relationships within the angiosperm order Caryophyllales were investigated by comparative sequencing of two portions of the highly conserved inverted repeat (totaling some 1100 base pairs) coinciding with the region occupied by ORF2280 in Nicotiana, the largest gene in the plastid genomes of most land plants. Data were obtained for 33 species in 11 families within the order and for one species each of Plumbaginaceae, Polygonaceae, and Nepenthaceae. These data, when analyzed along with previously published ORF (open reading frame) sequences from Nicotiana. Spinacia. Epifagus, and Pelargonium using parsimony, neighbor-joining, and maximum likelihood methods, reveal that: (1) Amaranthus, Celosia, and Froelichia (all Amaranthaceae) do not comprise a monophyletic group; (2) Amaranthus may be nested within a paraphyletic Chenopodiaceae; (3) Sarcobatus (Chenopodiaceae) is allied with Nyctaginaceae + Phytolaccaceae (the latter family excluding Stegnosperma but including Petiveria); and (4) Caryophyllaceae (with Corrigiola basal within the clade) are sister group to Chenopodiaceae + Amaranthaceae. Basal relations within the order remain obscure. Sequence divergence values in pairwise comparisons across all Caryophyllales taxa ranged from 0.1 to 5% of nucleotides. However, despite these low values, 23 insertion and deletion events were apparent, of which five were informative phylogenetically and bolstered several of the relationships listed above. A polymerase chain reaction (PCR) survey for ORF homolog length variants in representatives from 70 additional angiosperm families revealed major deletions, of 100 to 1400 base pairs, in 19 of these families. Although the ORF is located within the mutationally retarded inverted repeat region of most angiosperm chloroplast DNAs, this gene appears particularly prone to length mutation.