The mostly tropical Amaranthaceae and the largely temperate Chenopodiaceae constitute the most diverse lineage (c. 180 genera and 2500 species) of the core Caryophyllales. One prominent feature shared by Amaranthaceae and Chenopodiaceae is the frequent occurrence of C4-photosynthesis. Together they contain c. 50% of C4-species known among eudicots. We report the first results of a phylogenetic analysis based on rbcL and 26S sequence data so far including 60 species of Chenopodiaceae and 25 of Amaranthaceae representing all tribes of the two families. We use our phylogenies to identify the number of independently evolved C4-lineages in the two families, to estimate their age, and to interpret their evolution with reference to our present knowledge of their distribution and ecology, leaf anatomy and biochemistry. Major results are: (1) Polycnemoideae, traditionally classified with Chenopodiaceae, are sister to Amaranthaceae, whereas all other genera of Chenopodiaceae form the second major clade. Both families can thus be treated as monophyletic after transferring Polycnemoideae to Amaranthaceae. (2) Within Chenopodiaceae the Beteae branch first. Corispermeae, Suaedeae, Salicornioideae, Camphorosmeae incl. Sclerolaeneae and Salsoleae are monophyletic. All four groups belong to one lineage, with Suaedeae and Salicornioideae forming one clade, and Camphorosmeae and Salsoleae another one. Most representatives of Atripliceae and Chenopodieae are intermingled in a large monophyletic group. (3) Within Amaranthaceae subfamily Gomphrenoideae is monophyletic and nested within Amaranthoideae. Tribes and subtribes except Celosieae are not monophyletic.(4) In both families C4-photosynthesis originated repeatedly and at different times.

Key words: Amaranthaceae, C4-photosynthesis, Caryophyllales, Chenopodiaceae, phylogeny