THE DARK SIDE OF NGC-3109

New optical and radio observations of NGC 3109 (DDO 236) are presented. From the surface photometry in the I band, a scale length α-1 = 3.11′ is found for the exponential disk. Comparison with Previously published B photometry shows that the surface color remains nearly constant with radius out to 11′. Radio synthesis imaging in the H I spectral line reveals a gaseous envelope 40′ × 12′ in size. The H I distribution is severely warped in the outer parts, with a ∼5-kpc-long appendage on the southwest side. A tilted ring model of the observed velocity field yields a slowly rising rotation curve; neither the turnover point nor the flat portion are reached with the present observations. The maximum rotational velocity is only ≃67 km s-1 at the last measured point of the rotation curve. Studying the mass distribution, a "no dark-matter" model with constant maximum (script M sign/LB)* = 1.1 for the stellar disk and including the distribution of the H I fails to reproduce the observed rotation curve. Combining the luminous matter with a dark, isothermal halo, gives (script M sign/LB)* = 0.5 ± 0.2 for the disk, and a core radius rc = 6.7 ± 0.7 kpc with a one-dimensional velocity dispersion σ = 47.0 ± 2.0 km s-1 for the halo component. While those two parameters are not well constrained, the central density of the halo ρ0≃0.008 ± 0.001 script M sign⊙ pc-3 is much better defined. At the last measured point of the rotation curve, NGC 3109 is entirely dominated by dark matter. In fact, the contribution from the dark component dominates at nearly all radii; for this system, there is clearly no "disk-halo conspiracy". Finally, it is shown that for NGC 3109, the H I gas traces the dark matter distribution very well.