A total of 126 radon passive dosemeters were distributed in 63 dwellings on the island of Lanzarote (Canary Islands) to measure the indoor radon concentration in the period April-June 1994. The mean overall indoor concentration was 50 Bq.m-3 with a standard deviation of 17 Bq.m-3. Applying the conversion factor for the effective dose, recommended by ICRP 65, this results in a mean effective dose of 0.75 mSv.y-1. The mean radon concentration in a single-family house proves to be higher at the ground floor than in upper levels. The mean radon concentration obtained in bedrooms is higher than in living rooms, independently of the floor they are located at. Statistically significant differences in the mean radon concentration have been found depending on the soil permeability (P=0.001) and building materials used (P=0.006).