We present the results of V and I photometries for ~56,500 stars of NGC 3109 obtained from a 467.26(')2 wide-field observations. The internal extinction does not affect the determination of the magnitude of the tip of the red giant branch (TRGB) from which a distance modulus of (m - M)0 = 25.61 ± 0.10 (1.32 ± 0.06 Mpc) have been derived. A mean metallicity of the RGB stars have been found to be [Fe/H] = -1.84 ± 0.2 (Z = 0.0003 ± 0.0001). The profile of the star density distribution shows two exponentials with a break radius located at ~16' and ~4' in the direction of the semimajor and semiminor axes, respectively. An extended low surface brightness structure perpendicular to the disk of the galaxy has been found. The radial distribution of the stellar populations implies that the disk is formed by stars of all ages while the extended extraplanar structure lacks young stars. From the main characteristics of NGC 3109 we deduce that the extended structure could be (1) a true halo formed only by old, low-metallicity stars being a low-mass spiral galaxy, or (2) a posthalo epoch formed either by a merging with smaller galaxies or by a past encounter with a neighbor galaxy like Antlia. If the latter, the extended structure could be composed of an intermediate-age population. Based on observations made with the 100 inch Irénée du Pont Telescope operated at Las Campanas Observatory (Chile).