  To properly understand the physics of upper main sequence stars it is
particularly important to identify the origin of their magnetic fields.
Recently, we confirmed that magnetic fields appear in Ap stars of mass below 3
M_sun only if they have already completed at least approximately 30% of their
main-sequence lifetime. The absence of stars with strong magnetic fields close
to the ZAMS might be seen as an argument against the fossil field theories.
Here we present the results of our recent magnetic survey with FORS1 at the VLT
in polarimetric mode of a sample of A, B and Herbig Ae stars with previously
undetected magnetic fields and briefly discuss their significance for our
understanding of the origin of the magnetic fields in intermediate mass stars.
