Laboratory—germinated seedlings of 26 populations, representing three species of Prosopis and obtained from a wide geographic expanse bounded by the Oklahoma—Kansas border to the north and inclusive of continental Mexico to the South, were utilized in experimental ecology during 1961 and 1962. In germination, no population or species character was affected by temperature or light. The ultimate height of seedlings on an experimental, reconstituted soil was substantially greater than for plants on a natural, acid soil from eastern Texas. Ten—, 12—, 14—, and 16—hr photoperiods with some temperature control and 12—, 14—, and 16—hr photoperiods under two temperature regimes resulted in less maximum height and an earlier inception of dormancy in Oklahoma and Texas populations. More southerly populations were less influenced by photoperiod. All populations had significantly greater heights under warm nights (24 degrees C) than under cool nights (13 degrees C) at a day temperature of 30 degrees C. In an experimental garden, a gradient in mean height and date of dormancy inception was recorded, more northern populations having lower maximum height averages and earlier dormancy. Freezing promoted dormancy in the Mexican populations. Spring activity was primarily under the influence of temperature. An ecoclinal response to low temperature, correlated with latitude of origin, showed least frost resistance in Mexican populations. The northern distribution of Prosopis is probably limited by the severity of winter temperatures; the eastern distribution is probably limited by edaphic factors. Ecotypic differentiation over the northern part of its range adds weight to the historical evidence that mesquite has occupied its current distribution for a considerable period of time. Orderly evolutionary developments have undoubtedly resulted in ecological differentiation in diverse habitats and thus have effected the presented distribution of mesquite.