The mineralogical constitution of a lateritic ore dictates the type of metallurgical process that can be used to extract valuable metals such as nickel and cobalt. To provide needed mineralogical information on the interrelationships of the metals contained in domestic laterites, the Bureau of Mines awarded a grant to Denver Research Institute to study the transition-type, low-grade laterites found in northern California and southern Oregon. For comparison, limonitic-type laterites from New Caledonia and the Philippines were also studied. X-ray diffraction, electron microscopy, electron beam analysis, and conventional petrography were used to determine the mineralogical associations of nickel and cobalt in the laterites. Results showed that for both laterite types studied most of the nickel and minor amounts of cobalt are associated with the mineral geothite (feo(oh)). Most of the cobalt and minor amounts of nickel are associated with a mixed manganese oxide (wad) mineral.