We show that the microquasar LS I +61 ^ { \circ } 303 is running away from its birth place in a young complex of massive stars .
The supernova explosion that formed the compact object shot out the x-ray binary with a linear momentum of 430 \pm 140 M _ { \odot } km s ^ { -1 } , which is comparable to the linear momenta found in solitary runaway neutron stars and millisecond pulsars .
The properties of the binary system and its runaway motion of 27 \pm 6 km s ^ { -1 } imply that the natal supernova was asymmetric and that the upper limit for the mass that could have been suddenly ejected in the explosion is \sim 2 M _ { \odot } .
The initial mass of the progenitor star of the compact object that is inferred depends on whether the formation of massive stars in the parent stellar cluster was coeval or a sequential process .