  We investigate the effect of the quark-gluon to hadron transition on the
evolution of cosmological perturbations. If the phase transition is first
order, the sound speed vanishes during the transition, and density
perturbations fall freely. The primordial Harrison-Zel'dovich density
fluctuations for scales below the Hubble radius at the transition develop
peaks, which grow linearly with the wavenumber, both for the
hadron-photon-lepton fluid and for cold dark matter. The large peaks in the
spectrum produce cold dark matter clumps of $10^{-8}$ to $10^{-11} M_\odot$.
