The geohydrology of McMullen Valley, west-central Arizona, was investigated using geologic, geophysical, and hydrologic data and a numerical model of the groundwater system. Interpretation of geologic and geophysical information indicates that the main structure of McMullen Valley is a syncline that has been normal faulted on the southeast side. Basin fill that accumulated in the structural depression during late Miocene to Pleistocene time is the main aquifer and is divided into upper and lower units on the basis of lithologic information. The upper unit is a thin layer of coarse-grained sediments and generally is not saturated. The lower unit is 3,000 to 4,000 ft thick, includes a fine-grained facies in the upper 1,000 ft, and is the main source of water. The fine-grained facies is found in the southwest half of the basin and is further divided into upper and lower parts. The lower part of the fine-grained facies has: a higher percentage of silt and clay than the upper part, contains evaporites, does not yield water to wells, and separates the aquifer into shallow and deep systems. A numerical model was used to analyze the groundwater system for both steady-state and transient conditions. The transient model was used to analyze system response to pumping stress. The transient system is one of storage depletion, and water level declines are controlled by pumping and specific yield distributions. Water level declines are also influenced by hydraulic properties and areal extent of the fine-grained facies. Significant water level declines may extend to aquifer boundaries in most of the basin; in one area, impermeable boundary greatly influences declines. The location of the nearby boundary was estimated through gravity data modeling. Several hydrologic components, including hydraulic properties and areal extent of the fine-grained facies , storage properties, and aquifer boundaries, need better definition in order to develop a more accurate model of the groundwater system. (Lantz-PTT)