  The metal deficient (Z = Z_sun/41) Blue Compact Dwarf Galaxy (BCD) SBS
0335-052 was observed with ISOCAM between 5 and 17 mic. With a L_12mic/L_B
ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a
low-metallicity object. The mid-infrared spectrum shows no sign of the
Unidentified Infrared Bands, which we interpret as an effect of the destruction
of their carriers by the very high UV energy density in SBS 0335-052. The
spectral energy distribution (SED) is dominated by a very strong continuum
which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mic,
the SED can be fitted with a grey-body spectrum, modified by an extinction law
similar to that observed toward the Galactic Center, with an optical depth of
A_V~19-21 mag. Such a large optical depth implies that a large fraction (as
much as ~ 75%) of the current star-formation activity in SBS 0335-052 is hidden
by dust with a mass between 3x10^3 M_sun and 5x10^5 M_sun. Silicate grains are
present as silicate extinction bands at 9.7 and 18 mic can account for the
unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a
nearly primordial environment contains as much dust as galaxies which are 10
times more metal-rich. If the hidden star formation in SBS 0335-052 is typical
of young galaxies at high redshifts, then the cosmic star formation rate
derived from UV/optical fluxes would be underestimated.
