Cold hardiness in a strain of laboratory-rearedFrankliniella occidentalis(Pergande) that originally came from a commercial greenhouse was examined. Lethal times (LT) at sub zero temperatures were found for second-larval instars, first and second-pupal instars, and adults. LT50at −5°C was 56.3–62.6 h for all tested instars and did not differ significantly among ins tars, At −10°C significantly different LT50values of 32.1, 24.7, and 12.1–15.4 h were found for adults, second-larval instars, and pupal instars, respectively. Supercooling points were measured for adults and second-pupal instar and were found to be significantly different at −21.3°C and −25.1°C, respectively. Supercooling points of adults reared at +25°C did not differ significantly between rearing photoperiods of 16:8 and 8: 16 (L:D) h.F. occidentaliswas found not to survive the winter of 1988–1989 in outdoor mesh cages in Denmark. Furthermore, the tolerance to total submergence in water supplied with fertilizer was investigated in four instars ofF. occidentalis. LT50values of 45.3, 32.1–39.6, and 20.7 h were found for second larval instar, pupal instars and adults, respectively.