  We use X-ray observations of quasar microlensing (sensitive to smaller
compact objects than in the optical) to study the possible presence of a
population of low mass black holes (from $\sim$ $10^{-3}M_{\odot}$ to
$10^{-1}M_{\odot}$) in lens galaxies. We compare these observations with
microlensing magnification simulations of a mixed population of stars and black
holes (BHs) plus a smooth matter component. We estimate the individual mass
fractions of both, stars and BHs, for three different BH masses in the range of
substellar to planetary masses. Our Bayesian analysis indicates that the
contribution of BHs is negligible in the substellar mass range but that a
population of BHs of planetary mass (M $\lesssim$ $10^{-3}M_{\odot}$) could
pass unnoticed to X-ray microlensing. We provide new upper limits to the
contribution of BHs to the fraction of dark matter based on both, the quasar
microlensing data in the X-ray band, and our previous estimates in the optical
of intermediate-mass BHs with an additional upper limit at $M=3M_{\odot}$.
