Southeastern Australia is situated in a temperate zone with generally mild winters. The diamondback moth, Plutella xylostella L., can be a serious pest of canola as well as of other Brassica crops in this region. However, the ability of P. xylostella to overwinter in southeastern Australia, as well as in other temperate regions of the world, remains controversial. Laboratory experiments indicated that after 60 d in a temperature regimen alternating between 0 and 5°C, 32% of third- and fourth-instar larvae pupated, 38% remained as larvae, and 57% of pupae survived, whereas only 15–20% of adult moths were still alive. Second-, third-, and fourth-instar larvae and pupae survived -5°C for up to 2, 4, 6, and 13 d, respectively, with some adults surviving >20 d. Furthermore, adults that survived 60 d of exposure to 0–5°C and 20 d at -5°C reproduced normally without supplemental food. An experiment simulating outdoor overwintering with caged populations and field survey for natural overwintering populations of P. xylostella on volunteer canola (Brassica napus L.) and wild turnip (Rapistrum rugosum L.) plants indicated that development and reproduction continued on these host plants through the winter months. These empirical results provide evidence that P. xylostella is capable of overwintering on suitable host plants in southeastern Australia, and the overwintering populations may contribute significantly to local population levels in spring and summer seasons.