Aspects of the life-cycle, host-plant relationships and mortality factors of Phyllonorycter messaniella (Zell.), a recently introduced pest of ornamental oaks (Quercus spp.) and some other trees in Australia, are described and discussed from observations in Melbourne, Victoria, in 1977--78. There were 3 generations a year, and breeding took place throughout the year. There were marked shifts in food-plant preference in autumn and spring. Leaf-mine areas increased during the late summer generation, and mine densities increased on some hosts in summer. Parasitism by a complex of 8 chalcidoids, only 2 of which were common (Achrysocharoides sp. and a possible species of Sympiesis) averaged about 20% throughout the year. Seasonal variation in parasitism and other mortality factors are described in relation to the moth's rapid colonisation in Australia.