A new simple, sensitive and rapid catalytic kinetic method for the determination of trace amounts of molybdenum(VI) ion is described. The method is based on the catalytic effect of Mo(VI) on reduction reaction of Nile blue A (NBA) by hydrazine dihydrochloride (HD) in acidic media. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of the reaction mixture at 634 nm. The fixed time method was used for the first 3 min from initiation of the reaction. Under the optimum conditions, in the concentration range of 0.08-8 μg/ml of molybdenum, a quite linear curve (r=0.9991, n=13) was obtained. The detection limit is 60 ng/ml. The relative standard deviation of 10 replicate measurements is 1.6% for a 1-μg/ml solution of Mo(VI). The proposed method was applied successfully to the determination of Mo(VI) in plants.