Report carried out for CETESB in 2006 showed 80% of the 645 cities are totally or partially supplied by underground water, and for 13 river basins of São Paulo State the underground water is the priority source for potable water supply. The use of underground water in different sectors reinforces its importance and highlight the concern about their protection and quality. The northwest region of São Paulo state is mainly supplied by underground water of the Guarani aquifer extracted by tubular wells, which presents some occurrence of Selenium. In the present study it was looked and evaluated, through the comparison of high sensitivity and selectivity methods as the atomic absorption spectrometry using electrothermal atomizaton (GFAAS) and the hydride generation (HGAAS). Underground water samples of four deep tubular wells of Guarani aquifer in the northwest of São Paulo state were collected to measure the selenium element. In addition to selenium other metals were analyzed (B,