A new design of two-filter radon detector has been developed for measurement of extremely low levels of radon in the harsh environments on board ships and remote islands. These were needed for the First Aerosol Characterization (ACE 1) multiplatform experiment in the Southern Ocean. By employing an internal recirculation system and a wire mesh screen as the second filter it has been possible to reduce the power consumption by as much as a factor of 10 and the weight and cost by a factor of 2 compared to current designs of comparable sensitivity. A very high efficiency of 0.38 count s-1 Bq-1 radon in the instrument has been achieved by counting while sampling. This is a key parameter because the larger this number, the smaller the volume and power consumption of the detector. Two air flow paths are used to separate the high flow rate needed to prevent loss of radon daughters to the walls of the detector and the lower flow rate needed to change the air sample in the instrument. As a result, the inlet air lines and delay chamber needed to remove thoron are compact. With a volume of 750 L the detectors used on board ships for ACE 1 had a sensitivity of about 0.2 counts s-1 Bq-1 m-3 and a lower limit of detection of 40 mBq m-3 for a 1 hour count. An instrument with a volume of 10 m3 and incorporating improvements made since ACE 1 could be expected to have sensitivity of 3.7 c s-1 Bq-1 m-3 and a lower limit of detection of 2 mBq m-3. At 45 min the time resolution is twice as good as that of instruments using a low internal flow rate, but not as good as instruments with a moving filter, where the sampling period is precisely defined. Dual-flow loop radon detectors with screens have the virtues of simplicity and freedom from routine maintenance. This new technology extends the range of sites at which baseline radon measurements can be made to remote areas with little regular technical support and a harsh environment. ¿ 1998 American Geophysical Union