Some areas of the world, called high background radiation areas (HBRAs), have anomalously high levels of natural background  radiation and the population residing in the areas is exposed to higher levels of radiation doses than other parts of the world where the natural radioactivity contents are normal. In the present investigation, levels of radon, thoron and their progeny are studied in 110 houses in the coastal region of the Kollam district in the state of Kerala, India using the multidetector twin cup dosimeter. Among these, 10 houses were studied in detail with five dosimeters in each house. Radon activity concentrations were found to vary from 7 to 100 Bqm^-3 and that of thoron from 4 to 66 Bqm^-3 in Neendakara panchayat. In Chavara panchayat, the variations of radon concentrations were from 7 to 83 Bqm^-3 and thoron concentrations were varied from 4 to 86 Bqm^-3. The occurrence of radon and thoron concentrations in the dwellings for both study areas shows that in 50 % of the dwellings, the concentration of radon is about 25 Bqm^-3 and in 60 % of the dwellings thoron concentration is about 15 Bqm^-3. The ratio of thoron-to-radon concentrations in the dwellings showed a mean value 0.55 (GM50.45) for the region.