An outbreak of coccidioidomycosis among 22 Navy SEALs occurred during training exercises in Coalinga, California. Ten (45%) of the 22 men had serologic evidence of acute coccidioidomycosis, the highest attack rate ever reported for a military unit. All case patients were symptomatic, and 50% had abnormal chest radiographs. There were no cases of dissemination and no deaths to date. Coccidioidomycosis continues to be a threat to military members and civilians who reside or train in areas where Coccidioides immitis, the causative agent, is endemic.