Deep Galaxy survey at 6.75 mu m with the ISO satellite

Deep 6.75 mu m mid-IR ISOCAM observations were obtained from the Canada-Fra nce Redshift Survey (CFRS) 1415+52 field with the Infrared Space Observator y. The identification of the sources with optical counterparts is described in detail, and a classification scheme is devised which depends on the S/N of the detection and the inverse probability of chance coincidence. 83 % o f the 54 ISOCAM sources are identified with I-AB <23.5 counterparts. The (I - II)(AB) colors, radio properties, spectrophotometric properties and freq uency of nuclear activity of these counterparts differ on average from thos e of typical CFRS galaxies. CFRS spectra are available for 21 of the source s which have I-AB less than or equal to 22.5 (including 7 stars). Most of t he strongest sources are stars or AGN. Among the nonstellar counterparts wi th spectra, 40% are AGNs, and 53% are galaxies that display star formation activity and/or significant contributions of A stars. The ISOCAM sources al so display an IR excess, even when compared with heavily-reddened local sta rburst galaxies. An upper limit of 30% of extragalactic ISO sources could b e at z > 1 of the 44 S-6.75 mu m > 150 mu Jy sources which are non-stellar (7 "spectroscopic" and 3 "photometric" stars excluded).