Arsenic concentrations measured by graphite furnace atomic absorption range from < 5 to 900 ¿g/L in groundwater pumped from 6000 wells within a 25 km2 area of Bangladesh. The proportion of wells that exceed the Bangladesh standard for drinking water of 50 ¿g/L arsenic increases with depth from 25% between 8 and 10 m to 75% between 15 and 30 m, then declines gradually to less than 10% at 90 m. Some villages within the study area do not have a single well that meets the standard, while others have wells that are nearly all acceptable. In contrast to the distribution of arsenic in the 8--30 m depth range which does not follow any obvious geological feature, the arsenic content of groundwater associated with relatively oxic Pleistocene sand deposits appears to be consistently low. The depth of drilling necessary to reach these low-As aquifers ranges from 30 to 120 m depth within the study area.