The intention of the study was to establish the flow regime and quality of the groundwater, and to define the mechanisms of recharge, that prevail in the Hadejia-Jamaare-Komadugu-Yobe Basin, in semiarid northeastern Nigeria. This basin is quite significant, as at its centre is the Hadejia-Nguru Wetlands (HNW)-a wildlife conservation area-in addition to it being the most extensive floodplain in the region. Consequently, water level measurements, along with geochemical and stable isotope (delta O-18 and delta H-2) sampling at 150 groundwater points, were carried out across the basin. Results show that groundwater flows from the southwest to the northeast following the river system. Furthermore, the Yobe River acts as effluent in a short reach between the towns of Gashua and Geidam. Groundwater quality in the study area is generally good, although isolated areas of anthropogenic contamination from leakage of domestic waste and agriculture, as well as dissolution of evaporate minerals, have been identified. The stable isotopes have a wide range of values, from -7.4 to +3.4 parts per thousand delta O-18 and -49 to +14 parts per thousand delta H-2. Groundwater isotopic signatures regress along the relation delta H-2 = (5.5 center dot delta O-18 - 4.9) and indicate evaporation effects during recharge processes. The results clearly identify areas proximate to surface waters of the HNW and the rivers as having focused recharge, with enriched delta O-18 and delta H-2 values. The areas remote from surface water are characterised by diffuse recharge with depleted isotope values, where recharge is directly from precipitation infiltrating through the unsaturated zone.