  We discuss the nature of faint 6.7um galaxies detected with the mid-infrared
camera ISOCAM on board the Infrared Space Observatory (ISO). The 23 hour
integration on the Hawaii Deep Field SSA13 has provided a sample of 65 sources
down to 6uJy at 6.7um. For 57 sources, optical or near-infrared counterparts
were found with a statistical method. All four Chandra sources, three SCUBA
sources, and one VLA/FIRST source in this field were detected at 6.7um with
high significance. Using their optical to mid-infrared colors, we divided the
6.7um sample into three categories: low redshift galaxies with past histories
of rapid star formation, high redshift ancestors of these, and other star
forming galaxies. Rapidly star forming systems at high redshifts dominate the
faintest end. Spectroscopically calibrated photometric redshifts were derived
from fits to a limited set of template SEDs. They show a high redshift tail in
their distribution with faint (<30uJy) galaxies at z>1. The 6.7um galaxies tend
to have brighter K magnitudes and redder I-K colors than the blue dwarf
population at intermediate redshifts. Stellar masses of the 6.7um galaxies were
estimated from their rest-frame near-infrared luminosities. Massive galaxies
(M_star~10e11M_sun) were found in the redshift range of z=0.2-3. Epoch
dependent stellar mass functions indicate a decline of massive galaxies'
comoving space densities with redshift. Even with such a decrease, the
contributions of the 6.7um galaxies to the stellar mass density in the universe
are found to be comparable to those expected from UV bright galaxies detected
in deep optical surveys.
