Abstract A balloon-borne instrument for making far infrared sky surveys with 2° angular resolution is described. In two initial flights at a wavelength of 320 μm approximately half of the celestial sphere including most of the northern milky way was surveyed. The thermal emission of the moon was alone detected. The upper limit to the flux from other sources was 3 x 10-12 W cm-2 in the 300 to 360 μm band, or approximately 2 x 10-23 W cm-2 Hz-1. A blackbody (optically thick) source 2° or greater in diameter yielding this flux would have a temperature of 10 °K. A warmer, small or optically thin source providing this much radiation in the Rayleigh-Jeans tail of the Plank distribution would have a temperature averaged over the 2° beam of 0.6 °K. These observations can be used to set upper limits to the opacity and temperature of interstellar grains.