Context : The general context of this paper is the study of magnetic fields in the pre-main sequence intermediate mass Herbig Ae/Be stars .
Magnetic fields are likely to play an important role in pre-main sequence evolution at these masses , in particular in controlling the gains and losses of stellar angular momentum .

Aims : The particular aim of this paper is to announce the detection of a structured magnetic field in the Herbig Ae star HD 190073 , and to discuss various scenarii for the geometry of the star , its environment and its magnetic field .

Methods : We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to obtain high-resolution and signal-to-noise circular polarization spectra which demonstrate unambiguously the presence of a magnetic field in the photosphere of this star .

Results : Nine circular polarization spectra were obtained , each one showing a clear Zeeman signature .
This signature is suggestive of a magnetic field structured on large scales .
The signature , which corresponds to a longitudinal magnetic field of 74 \pm 10 G , does not vary detectably on a one-year timeframe , indicating either an azimuthally symmetric field , a zero inclination angle between the rotation axis and the line of sight , or a very long rotation period .
The optical spectrum of HD 190073 exhibits a large number of emission lines .
We discuss the formation of these emission lines in the framework of a model involving a turbulent heated region at the base of the stellar wind , possibly powered by magnetic accretion .

Conclusions : This magnetic detection brings an important element for our understanding of stellar magnetism at intermediate masses .