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The study was undertaken with the aim of estimating vulnerability index of shallow aquifer within weathered crystalline regolith that overlies the basement complex rocks of Oke-lla, Osun State, South-western Nigeria. Twenty five vertical electrical soundings adopting Schlumberger configuration were used to investigate the subsurface lithology in an area covering 48 km2. The result revealed four distinct geologic layers which consist of top soil, weathered layer (clayey/sandy saprolite), sand, and fractured/fresh basement rocks. The saprolite, characterized by resistivity in the range of 44 and 471 Ωm with thickness varying from 7 to 16 m, acts as shallow aquifer storing infiltration water. The thickness of the layers above the aquifer, as obtained from quantitative interpretation of resistivity sounding data and estimates of hydraulic conductivities, were used to quantify vulnerability indices. The obtained aquifer vulnerability index shows that, in 70% of the study area, the aquifer has high to extremely-high vulnerability and may be vulnerable to effluents discharge that percolate into the aquifer tapped by hand-dug wells for domestic purposes.