A review is presented of the initial importation of the Australian mealybug lady beetle,Cryptolaemus montrouzieriMulsant, in 1891–92 from mild climatic areas of eastern Australia, and the subsequent history of its periodic releases against mealybugs in California. Since its introduction, this predator, which can rarely survive California winters, has been maintained contmuously in insectary culture and redistributed on a world-wide basis without further additions to its gene pool. New clones of the predator, collected in 1972 in 2 of the coldest areas of Australia, were propagated and colonized in California to try to broaden the genetic base for cold-hardiness of the species.Special equipment was designed for laboratory testing of the cold tolerance of this predator. In tests with this equipment at –5.0°C for less than 12 h, the old (1891) race ofC. montrouzieriwas shown to be less cold-hardy than the newly imported biotypes. Evidence is supplied that a simple test of adult survival after 12–h exposure to –5.5±0.5°C affords an easy procedure for measuring the cold-hardiness of this and other natural enemy species.