  We report the detection of faint emission in the high-excitation [OIV]
25.90um line in a number of starburst galaxies, from observations obtained with
the Short Wavelength Spectrometer (SWS) on board ISO. Further observations of
M82 spatially resolve the [OIV] emitting region. Detection of this line in
starbursts is surprising since it is not produced in measurable quantities in
HII regions around hot main-sequence stars, the dominant energy source of
starburst galaxies.
  We discuss various models for the formation of this line. [OIV] that is
spatially resolved by ISO cannot originate in a weak AGN and must be due to
very hot stars or ionizing shocks related to the starburst activity. For
low-excitation starbursts like M82, shocks are the most plausible source of
[OIV] emission.
