The uniparental parasitoid Closterocerus chamaeleon (Girault) is discovered to be fortuitously present on a population of the invasive Eucalyptus Gall Wasp Ophelimus maskelli (Ashmead) in Riverside, California. This is the first report from the New World of Closterocerus chamaeleon, which has proven to be a highly effective natural enemy of Ophelimus maskelli in the Mediterranean Basin. The taxonomy and identification of Closterocerus chamaeleon is discussed.