A field-dependent charge-carrier trapping process has been observed in the photoconduction mechanism of phenanthrene-pyromellitic acid dianhydride. This effect can be observed at electric field strengths exceeding 103 V/cm. A calculation on the basis of a hopping model shows that field-induced charge-carrier trapping occurs in a three-dimensional system at critical field strengths well above 106 V/cm. In a one-dimensional model, however, the critical field strength can be as low as 1 V/cm. © 1975 The American Physical Society.