1 European Southern Observatory, Garching bei Muenchen, Germany
2 European Space Agency, ESTEC, Noordwijk, Netherlands
ISOCAM imaged at 15 
five fields in the Large Magellanic Cloud
(LMC)
suspected to contain ultracompact HII regions. The selection of fields was
based on the existence of sources unresolved by IRAS with far-infrared colors
typical of galactic ultracompact HII regions. Given the small linear size of
these objects, the IRAS beam was heavily contaminated by unrelated LMC
sources, making color selection criteria much less reliable than in our
Galaxy. The dramatic improvement in angular resolution provided by ISOCAM has
allowed us to identify true compact objects in some of the fields, yielding
important clues on their nature and morphology.
Three bright, unresolved sources appear in two of our ISOCAM fields. They belong to the HII complex N159, near the 30 Doradus region. One of them is identified with a known compact radiosource, while for the other two no compact radio counterpart has been detected. The lack of radio continuum emission from a point source at 2.4 GHz indicates an infrared to radio continuum flux ratio much higher than in typical compact HII regions, especially considering that the lower metallicity of the LMC, and consequently lower dust contents, should reduce that ratio with respect to typical values in our Galaxy.
This large ratio suggests that the two point sources in question are true
ultracompact HII regions. The lack of point source emission at 2.4 GHz can
be explained by a very high electron density of the emitting region,
characteristic of ultracompact HII regions, making them optically thick at
frequencies above 
5 GHz. The non-detection of compact radio continuum
sources at the positions of the bright infrared point sources, despite of their
brightness at 15 ,
can thus be taken as an indication of them being
optically thick at that frequency.
The angular resolution of the radio and infrared observations separately is insufficient to reliably pinpoint bona-fide ultracompact HII regions at the distance of the LMC. Nevertheless, our observations demonstrate how their combination can provide insight on the physical conditions of the emitting volume. In this way, the two radio-quiet bright point sources detected by ISOCAM can be considered as the best ultracompact HII region candidates identified so far in a galaxy different from ours. This provides a first sample of objects suitable for future studies of the early stages of massive star formation in an environment of much lower metallicity than that of our Galaxy.