We present results from an experiment using an array of scintillators and time digitizing techniques to search in cosmic rays for grand unification magnetic monopoles and other ionizing heavy particles at the Earth’s surface. An upper flux limit of 4.7×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\sec }^{\mathrm{-}1}$ for 2.7×${10}^{\mathrm{-}4}$≤β≤5×${10}^{\mathrm{-}3}$ (90% C.L.) for such particles having an ionization ≥(1/3 of minimum ionizing particles has been obtained. These results do not improve on the limits from underground experiments for heavy monopoles or other particles (M∼${10}^{16}$ GeV); however, they provide new flux limits for somewhat lighter particles (M∼${10}^9$ GeV) which cannot penetrate deep underground or come with large zenith angles. Our result implies that such electrically or magnetically charged particles have been excluded as the major component of the dark matter of the Universe.

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