A new system for on-line preconcentration of molybdenum by sorption on a minicolumn associated to inductively coupled plasma -optical emission spectrometry with ultrasonic nebulization was studied. It is based on the sorption of molybdenum on a column packed with immobilized baker's yeasts on controlled pore glass without further complexing reagent. The molybdenum preconcentrated by biosorption was subsequently eluted with hydrochloric acid. Considering a sample flow rate of 5.0 mL min -1 , 10 mL of sample was preconcentrated in 2 min. achieving a sensitive total enhancement factor of 480-fold, and t he detection limit (3s) obtained was 21 ng L -1 .Additionally, the calculated precisions expressed as percent relative standard deviation (RSD%) was 1.9%.Satisfactory results were obtained for the determination of molybdenum in standard reference material NIST 1643e Trace Elements in Water and real water samples.