2-Acetylthiophene isonicotinoylhydrazone (ATINH) has been employed for the direct, non-extractive spectrophotometric determination of mercury. The reagent instantaneously forms a green yellow coloured species (1:2 Hg-ATINH complex) with mercury(II) in acidic medium (pH 6.0, sodium acetate-acetic acid buffer) which remains constant for more than 12hours. The species exhibits maximum absorbance at 420 nm ,with a molar absoprtivity of 4. 1 x 10³ L. mol^-1 cm^-1. Beer's law is valid over the concentration range of 3.2-32.0 ppm of Hg(II). The Sandell's sensitivity of the complex is 0.875 g cm^-2 of Hg(II). Large number of cations and anions and complexing agents do not interfere in the determination of mercury. The method was successfully applied to biological and soil samples.