Wide field BVI photometry and $^{12}$CO(1$\to$0) observations are presen ted in the region of the open cluster NGC 6520 and the dark molecular cloud Barnard~86. From the analysis of the optical data we find that the cluster is rather compact, with a radius of 1.0$±$0.5 arcmin, smaller than previous estimates. The cluster age is 150$±$50 Myr and the reddening E$_{B-V}$=0.42$±$0.10. The distance from the Sun is estimated to be 1900$±$100 pc, and it is larger than previous estimates. We finally derive basic properties of the dark nebula Barnard 86 on the assumption that it lies at the same distance of the cluster. ; Comment: 21 pages, 8 eps figures (a few degraded in resolution), accepted for publication in the Astronomical Journal