Indoor radon concentration level was measured in twelve selected phosphate fertilizer warehouses in Nigeria in order to establish potential hazards to persons using such warehouses as offices. The fertilizer warehouses were selected based on the brand of fertilizers stored, size, ventilation pattern and the number of workers in the warehouses during working hours. Electret Ion Chamber Technology (EIC) with the trade name E-PERMTM was employed for the measurement of radon concentration in the warehouses. Average radon concentration in the warehouses range between 33.6Bqm−3 and 117Bqm−3with an arithmetic mean of 91.62±5.9Bqm−3.