  We present a preliminary measurement of the q2 dependence of the D0 --> K- e+
nu_e decay rate. This rate is proportional to the hadronic form factor squared,
specified by a single parameter. This is either the mass in the simple pole
ansatz m_pole = (1.854 +- 0.016 +- 0.020) GeV/c2 or the scale in the modified
pole ansatz alpha_pole = 0.43 +- 0.03 +- 0.04. The first error refers to the
statistical, the second to the systematic uncertainty.
