  Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active
galactic nuclei (AGN). They have demonstrated that the presence of relativistic
jets in an AGN is not strictly related to its radio-loudness, the black hole
mass, or the host galaxy type. Here we present a remarkable example of a
radio-quiet NLS1, 6dFGS gJ035432.8-134008 (J0354-1340). In our Karl G. Jansky
Very Large Array observations at 5.5 GHz the source shows a bright core with a
flat spectral index, and extended emission corresponding to very elongated
jets. These are the largest double-sided radio jets found to date in an NLS1,
with a deprojected linear size of almost 250 kpc. We also analysed
near-infrared and optical images obtained by the Magellan Baade and the
European Southern Observatory New Technology Telescope. By means of photometric
decomposition and colour maps, we determined that J0354-1340 is hosted by a
spiral/disk galaxy. Fully evolved relativistic jets have traditionally been
associated with high-mass elliptical galaxies hosting the most massive black
holes, but our results confirm that also less massive black holes in spiral
galaxies can launch and sustain powerful jets, implying that the launching of
the jets is governed by factors other than previously believed.
