A new class of very cold young stellar objects (YSOs) has recently
 been identified and designated "Class 0", which were all
 undetected by IRAS at 12 microns. These cold YSOs
 emit the bulk of their luminosity in the submillimeter
 range and correspond to the youngest protostars known
 to date. We propose to conduct deep, high angular resolution
 observations of the envelopes of these cold protostars
 with ISOCAM at 7.75 and 15 microns, wavelengths where the diffuse
 interstellar background is relatively intense and
 extinction at a minimum. This will allow us
 (1) to check the hypothesis that Class 0 protostars can
 be seen in absorption at mid-IR wavelengths and are
 therefore qualitatively different from the more
 evolved Class I sources detected in near-IR surveys, and
 (2) to investigate the spatial structure of their circumstellar
 envelopes with more accuracy than is presently possible
 in the submillimeter. If achieved, this second objective
 will set strong constraints on star formation theories,
 which make definite predictions about the density
 gradient of protostellar cores/envelopes. In order to supplement
 this spatial study at far-IR wavelengths (where Class 0 protostars
 should be detectable in emission with ISO), we also propose
 super-resolution PHOT observations of one of these sources (not
 included in the central programme) at 60 and 90 microns.