We have obtained V- and I-band photometry for about 17,500 stars in the field of the dwarf irregular galaxy NGC 3109, located in the outskirts of the Local Group. The photometry allows us to study the stellar populations present inside and outside the disk of this galaxy. From the VI color-magnitude diagram, we infer metallicities and ages for the stellar populations in the main body and in the halo of NGC 3109. The stars in the disk of this galaxy have a wide variety of ages, including very young stars of age ~107 yr. Our main result is to establish the presence of a halo consisting of Population II stars, extending out to about 45 (or 1.8 kpc) above and below the plane of this galaxy. For these old stars we derive an age of over 1010 yr and a metallicity of [Fe/H] = -1.8 ± 0.2. We construct a deep luminosity function, obtaining an accurate distance modulus (m - M)0 = 25.62 ± 0.1 for this galaxy based on the I-magnitude of the red giant branch tip and adopting E(V-I) = 0.05.
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