  The Infrared Space Observatory was used to search for a tracer of the warm
and dense neutral interstellar medium, the [O I] 63.18 micron line, in four
ultraluminous IRAS sources lying at redshifts between 0.6 and 1.4. While these
sources are quasars, their infrared continuum emission suggests a substantial
interstellar medium. No [O I] flux was securely detected after probing down to
a 3 sigma sensitivity level sufficient for detecting line emission in
starbursts with similar continuum emission. However, if the detection threshold
is slightly relaxed, one target is detected with 2.7 sigma significance. For
this radio-quiet quasar there is likely a substantial dense and warm
interstellar medium; the upper limits for the three radio-loud sources do not
preclude the same conclusion. Using a new, uniformly-processed database of the
ISO extragalactic far-infrared spectroscopy observations, it is shown that
nearby Seyfert galaxies typically have higher [O I]-to-far-infrared ratios than
do normal star-forming galaxies, so the lack of strong [O I] 63 micron emission
from these high-redshift ultraluminous sources cannot be attributed to their
active cores.
