Systematic analysis of the stable carbon isotopic composition of fossil land plants (delta C-13(p)) has the potential to offer new insights regarding paleoclimate variation and plant-environment interactions in early terrestrial ecosystems. delta C-13(p) was measured for 190 fossil plant specimens belonging to 10 genera of Early to Late Devonian age (Archaeopteris, Drepanophycus, Haskinsia, Leclercqia, Pertica, Psilophyton, Rhacophyton, Sawdonia, Tetraxylopteris, and Wattieza) and 2 genera of Early Carboniferous age (Genselia and Rhodeopteridium) collected from sites located mainly in the Appalachian Basin (22-30 degrees S paleolatitude). For the full carbon-isotopic dataset (n = 309), delta C-13(p) ranges from -20.3 parts per thousand to -30.5 parts per thousand with a mean of -25.5 parts per thousand, similar to values for modern C-3 land plants. In addition to a secular trend, delta C-13(p) exhibits both intra- and intergeneric variation. Intrageneric variation is expressed as a small (mean 0.45 parts per thousand) C-13-enrichment of leaves and spines relative to stems that may reflect differential compound-specific compositions. Intergeneric variation is expressed as a much larger (to similar to 5 parts per thousand) spread in the mean delta C-13(p) values of coeval plant genera that was probably controlled by taxon-specific habitat preferences and local environmental humidity. Among Early Devonian taxa, Sawdonia yielded the most C-13-depleted values (-27.1 +/- 1.7 parts per thousand), reflecting lower water-use efficiency that was probably related to growth in wetter habitats, and Leclercqia, Haskinsia, and Psilophyton yielded the most C-13-enriched values (-23.0 +/- 1.6 parts per thousand, -22.3 +/- 1.3 parts per thousand, and -24.8 +/- 1.6 parts per thousand, respectively), reflecting higher water-use efficiency probably related to growth in drier habitats.