We measured ortho-positronium (o-Ps) formation probability, I-3 in amorphous SiO2 as a function of external electric field up to 100kV/cm. It was found that I, decreases from 53.9% at F = 0 to 48.7% at F = 100kV/cm. In spite of the strong electric field effect on the Ps yield, the positron mobility, determined by the positron annihilation Doppler broadening technique, was 0 +/- 2cm(2)/s.V in this material. Therefore, a-SiO2 is the first ever studied bulk solid material with low positron mobility, exhibiting strong electric field effect on Ps formation. The field dependent Ps formation in a-SiO2 is associated with the high mobility of excess electrons but not that of the positron as in the case of nonpolar polymers such as polyethylene.