This paper reports on the occurrence of methyl tert-butyl ether (MTBE) in groundwater. As part of the US Geological Survey's National Water Quality Assessment Program, concentrations of at least 60 volatile organic compounds (VOCs) were measured in samples from 2,743 wells across the United States between 1993 and 1998. The detection frequencies for 59 of these VOCs were normalized at a censoring level of 0.2 ug/L for all samples to allow detection frequencies to be compared. However, samples collected after 1995 had VOC detections reported at concentrations less than 0.2 ug/L. These lower detection levels result in an increase in the detection frequencies for many VOCs, such as MTBE. Wells were sampled to assess the ambient groundwater; areas where point-source contamination was known to exist were excluded from this assessment. At a censoring level of 0.2 ug/L, MTBE was detected in 13.1% of 482 wells located in urban areas (population densities greater than or equal to 386 people/km2). MTBE was detected in 3.6% of 2,261 wells in rural areas (population densities less than 386 people/km2). In those areas sampled after 1995, detection frequencies of MTBE at least doubled when concentrations of MTBE less than 0.2 ug/L were compared to detection frequencies at a censoring level of 0.2 ug/L. MTBE was the second most frequently detected VOC in urban and rural areas. The most frequently detected VOC was trichloromethane (chloroform) and the third most frequently detected VOC was tetrachloroethene (PCE) in both of these areas.