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Soil samples collected from five states in the southwestern part of Nigeria have been analyzed for activity concentration of gamma-ray emitters. The States were Oyo, Lagos, Ondo, Ogun, and Osun. Activity concentration measurement was carried out using a co-axial type high purity germanium (HPGe) detector. Spectral analyses were performed using spectrometry software. The activity concentration measured ranged from 0.03 ± 0.2 Bq kg−1 for 137Cs to 1,358.6 ± 28.5 Bq kg−1 for 40K. 40K accounted for most of the activity. The calculated absorbed dose rates in air at 1.0 m above the ground ranged from 6 to 303 nGy h−1 with a mean and standard deviation of 60.5 nGy h−1 and 63.2 nGy h−1, respectively. Estimated annual outdoor effective dose equivalent varied from 8 to 370 mSv y−1. The calculated collective dose equivalent values for the five states ranged from 87,000 person-Sv to 600,000 person-Sv. The estimated annual outdoor effective values were considerably higher than the world soil average value of 70 μSv y−1 reported by the International Commission on Radiological Protection.