The process of accretion onto the isolated black holes under the various conditions of the ISM and its observational manifestations are discussed. For the majority of the Galaxy volume the accretion rate is as low as 10−6–10−9 of Eddington one, and the accretion is spherically-symmetric. Such objects manifest itself as a weak optical and x-ray sources with featureless spectra and the significant variability of the emission.
For the BH located inside the dense molecular cloud the regime of accretion depends on its massand velocity. A massive (100–1000M⊙) BH born in the cloud or having low relative velocity, may manifest itself as a ultra-luminous x-ray source (ULX).