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Ultramafic cumulate pods occur locally just below the base of the sheeted dike complex in the Josephine ophiolite in north‐western California. These ultramafic rocks occur with metagabbros, and both are extensively intruded by mafic dikes. The ultramafic cumulates contain evidence of high‐temperature hydrothermal alteration (early alteration possibly above 900°C), which continued to progressively lower temperatures (<350°C). This hydrothermal alteration has been partially obscured by later low‐grade regional metamorphism. Petrographic evidence for the hydrothermal alteration includes (1) hydration of primary clinopyroxene to hornblende, (2) supersolvus secondary hornblende, (3) subsolvus coexisting hornblende + actinolite and actinolite + chlorite, (4) alteration of hornblende to chlorite, (5) alteration of orthopyroxene and olivine to serpentine, and (6) serpentine veining. The composition of the altering fluid appears to change with temperature.