An empirical relationship is developed between the difference of vertically and horizontally polarized brightness temperatures (ΔT) observed at 37 GHz frequency of the Scanning Multichannel Microwave Radiometer (SMMR) on board the Nimbus-7 satellite and primary productivity over hot arid and semi-arid regions of Africa and Australia. The primary productivity values for this empirical relationship are calculated according to Leith (1975). The empirical relationship is then used, together with the observed ΔT, to estimate primary productivity over a roughly 60000 km2area in the Thar Desert from 1979 to 1985. The highest areally averaged productivity of 0.368kgm−2yr−1is estimated for 1979, while the lowest 0.222kgm−2yr−1for 1985; the average for the 7 years being 0.271 kgm−2yr−1. The spatial variability of the productivity values within the study area is found to be quite substantial; the standard deviation about the mean being about 008kgm−2yr−1.