We have observed the cluster Abell 2218 ( z = 0.175 ) with ISOCAM on board the Infrared Space Observatory using two filters , LW2 and LW3 , with reference wavelengths of 6.7 and 14.3 \mu m , respectively .
We detected 76 sources down to 54 and 121 \mu Jy ( 50 % completeness levels ) at 6.7 and 14.3 \mu m , respectively .
All these sources have visible optical counterparts .
We have gathered optical and near-infrared magnitudes for 60 of the 67 non-stellar optical counterparts to the ISOCAM sources , as well as redshifts for 43 of them .
We have obtained acceptable and well constrained fits to the observed spectral energy distributions ( SEDs ) of 41 of these sources , using the “ GRASIL ” models of Silva et al .
( 1998 ) , and have determined their total infrared luminosities ( L _ { IR } ’ s ) and star formation rates ( SFRs ) .

The SEDs of 20 ( out of 27 ) ISOCAM cluster members are best fit by models with negligible ongoing star formation , and no major episode of star formation in the last \sim 1 Gyr .
Their SEDs resemble those of 5–10 Gyr old early-type galaxies .
A slightly higher , but still very mild , star-formation activity is found among the remaining cluster sources , which are mostly spirals .
The median IR luminosity of the 27 ISOCAM cluster sources is L _ { IR } = 6 \times 10 ^ { 8 } L _ { \odot } . The ISOCAM-selected cluster galaxies have indistinguishable velocity and spatial distributions from those of the other cluster galaxies , and do not contribute significantly to the Butcher-Oemler effect .
If A2218 is undergoing a merger , as suggested by some optical and X-ray analyses , then this merger does not seem to affect the mid-infrared properties of its galaxies .

The SEDs of most ISOCAM-selected field sources are best fit by models with moderate ongoing star formation , with a significant fraction of their stellar mass formed in the last \sim 1 Gyr .
Their SEDs resemble those of massive star-forming spirals or starburst galaxies , observed close to the maximum of their star formation activity , but not necessarily during the short-lived starburst event .
The median redshift of these field galaxies is z \simeq 0.6 .
Their L _ { IR } ’ s span almost two orders of magnitudes , from \sim 10 ^ { 10 } L _ { \odot } to \sim 10 ^ { 12 } L _ { \odot } , with a median of 1.2 \times 10 ^ { 11 } ( eight of the 14 field sources are LIRGs ) .
The SFRs of these 14 ISOCAM-selected field sources range from 2 to 125 M _ { \odot } { \mbox { y } r } ^ { -1 } , with a median value of 22 M _ { \odot } { \mbox { y } r } ^ { -1 } .

We compare our findings with those obtained in other ISOCAM cluster and field surveys .