This paper presents a focal determination routine developed for use in locating microearthquakes recorded on a small tripartite seismic array. The method uses P-wave arrival times at four stations and the S-P time measured at one station. A solution is obtained for each trio of stations. These solutions can be compared to obtain an indication of the precision of the location. Practical problems such as velocity layering and station corrections, and a number of applications are discussed. The advantage of the method is that it leads directly to improved information about local structure.