ARMING by smallholders in arid northern Mexico has historically been limited by a shortage of land. In general, well-watered agricultural land is not abundant, and it either is owned by large-scale farmers or is controlled by the government.1 The smallholders have minimum access to this type of land, and they have two options when increased production is needed: to intensify activities on land already in use, or to expand cultivation to poorly watered land. Intensification is pursued in some cases, but smallholders usually encounter land-tenure and monetary constraints.2 With the exception of the Sistema Alimentario Mexicano (SAM), an integrated program for the accelerated production of basic foodstuffs that began in 1980 but was abandoned early in 1983 because of funding problems, the central government has been guided by the prevalent belief that lands not currently under cultivation are marginal and cannot be economically improved.3 Preference has been given to the development of large-scale mechanized agriculture and off-farm employment in urban areas rather than to the cultivation of marginal lands from which the return is slightly greater than production cost. Expansion of cultivated hectarage has not been encouraged in Mexico, but it is the option pursued by many smallholders. It will probably be used more frequently as demographic and urban problems worsen. The farmers who cultivate temporales, the fields that are dependent on direct precipitation or runoff, in bottom lands of arroyos near the pueblo or town of Baviacora in the valley of the Rio Sonora in eastern Sonora are only one group of Mexican smallholders who are expanding agriculture into marginal lands. The pueblo of Baviacora, with a population of 2,297 in 1980, is located in a municipio (the equivalent of a county in the United States) of the same name