It was hypothesized that the northward spread of the eucalyptus longhorned borer,Phoracantha semipunctataF., in California is limited by winter temperature conditions. The cold tolerance of prepupal stages was tested by exposing infested logs to cold temperature treatments of −5,0, +5, and +10°C for periods of 1, 3, 7, and 30 d. Temperature treatments had no affect on survivorship to adulthood or longevity but did reduce adult weight relative to nontreated control logs. Increasing exposure time to all cold temperatures significantly reduced both survivorship and adult weight but had no affect on adult longevity. Long periods of exposure to cold temperatures delayed emergence of adults and resulted in more closely synchronized emergence. Cold temperatures are unlikely to prevent the spread of this pest into most zones occupied by its eucalyptus host plants. In addition, increasing densities of prepupae in logs also resulted in significantly reduced survivorship to adulthood but did not affect adult weight or longevity.