We report experimental results on heterodyne holographic microscopy of subwavelength-sized gold particles. The apparatus uses continuous green laser illumination of the metal beads in a total internal reflection configuration for dark-field operation. Detection of the scattered light at the illumination wavelength on a charge-coupled device array detector enables 3D localization of brownian particles in water. OCIS : 180.6900, 090.1995, 170.0180 Assessing microscopic processes by tracking optical labels has a broad range of applications in biology. In particular, monitoring biological phenomena such as cellular-level dynamics is a subject of growing interest. Fluorescent molecules are widely used in this aim, but the observation time of dyes is limited by photobleaching. Quantum dots offer a much better photostability, but they have the inconvenient to blink. Noble metal nanoparticles have the advantage of being perfectly photostable [1]. Originally, light scattered by small metallic particles has been detected in dark field [1] or total internal reflection [2] configuration. To improve the detection sensitivity, interferometric approaches have been introduced [3,4,5,6,7]. Common-path interference, initially achieved with a Nomarski interferometer [8], translates phase variations into intensity variations, and enables the detection of phase perturbation provoked by spatial [9] or photothermal [10] modulation. Scanning heterodyne detection of the photothermal modulation [3] enables an unmatched combination of sensitivity and selectivity suitable to discriminate particles smaller than 5 nm from their background. Photothermal imaging relies on a spatial scanning of the laser beams to track the index modulation in the neighborhood of the beads; the temporal noise in the incident light leads to spatial noise in the image acquired sequentially [7]. Wide-field detection schemes alleviate such issues, but their sensitivity in optical mixing configurations would not match heterodyne detection on single detectors levels. We propose here a wide-field, shot-noise limited, tunable CCD heterodyne detection technique able to achieve high-resolution, 3D microscopy of gold particles with a laser source at rates compatible with biological dynamics. The experimental setup is sketched in fig. 1(a).The main laser beam (λ = 532 nm, field E L , 50 mW, single axial mode, CW) is split with a polarizing beam splitter (PBS) in local oscillator (LO) and illumination arms (fields E LO and E I ). A λ/2 waveplate (HWP), and neutral densities (A1, A2) allow control of the optical power in each arm. Both beams are frequencyshifted around 80 MHz by acousto-optical modulators (AOM) driven at frequencies ω AOM1 , ω AOM2 .The LO beam passes through a beam expander (BE) to form a plane wave which polarization angle is adjusted with a HWP to maximize the holographic modulation depth. The object is illuminated in dark field configuration by using total internal reflection (TIR). The scattered field E ≪ E LO passes through a microscope objective (MO...