Lepidopter infestation of Northern Sardinian cork-oaks was studied from 1964 to 1975. Data were obtained on population composition, factors influencing natural mortality, fluctuation rythms of the more important species, and on plant reaction to defoliation.67 species were observed, of which the most frequently noted phytophagous lepidopters were: (1)Dryobotodes eremitaFabr. and D.MonochromaEsp., - with latent fluctuation; (2)Tortrix viridanaL.,Ortollosia crudaSchiff. andSpudaea ruticillaEsp., - with temporary fluctuation; and (3)Phalera bucephalaL.,Malacosola neustriaL. andPorthetria dispar(L.), - with periodic fluctuation. P.dispar(L.) and M.NeustriaL. fluctuations occurred in cycles of approximately 9 years. Severe defoliation was observed in 1973 and 1974,Malacosomareaching maximum population density a year earlier than Porthetria.Numerous natural mortality factors affected the population fluctuation ofPorthetria. Particularly significant was the impact of predators and parasites on the pre-imaginal stages as well as that of pathogenetic agents (e.g. viruses, protozoa), although the effect of the latter varied from year to year and was not entirely related to larval population density. Apart from a 7- 10% sterility, eggs showed a mortality rate varying from 2.7% to 17.7% due to the parasitesOoencyrtus kuwanai(How.) (Hym., Encyrtidae) andAnastatus disparisRuschka (Hym. Eupelmidae) and the predatorsDermestes lardariusL. (Col. Dermestidae) andHaplocnemus jejunusKiensenw. (Col. Melyridae).The numbers of parasites on the larvae were in direct proportion to the larval population density and varied from 4.8% to 80.5 %; the most frequently encountered wereSturmia scutellataR.D.,Exorista segregataRond.,Exorista larvarumL. (Dipt. Tachinidae) andSarcophagidae of AgriaandSarcophaga genera. The predatorCalasoma sychophantaL. (Col. Carabidae) appeared in large numbers the year following a maximum population density of the host larva. In addition to the forementioned dipters, the following were also observed to emerge from thePorthetriachrysalides:Pales pavidaMeig. (Dipt. Tachinidae),Muscina stabulansFall. (Dipt. Muscidae),Pimpla instigatorF.,Theronia atalantaePoda (Hym., Ichneumonidae) andBrachymeria intermedia(Nees) (Hym., Chalcididae).Porthetriapopulation fluctuations were also affected by ambiental factors and by a reduction in fecundity when population densities were at their highest.Malacosoma neustriaL., apart from being subject to the same parasites and pathogenetic agents asPorthetria, was particularly influenced byPales pavidaMeig. andScambus malacosomaeSeyrig (a hymenopter parasite specific toMalacosomalarvae in cocoon).Gradations in the two defoliator populations are, therefore, regulated by a whole number of contemporary factors in which tachinids, ichneumonids and disease play the predominant role.Provided that spring rainfall exceeds 180 mm, completely defoliated cork-oaks recover their foliage within 15 to 20 days. The damage caused by parasitic attack is, thus, limited to a certain extent.As a result of the observations made on the dynamics and regulating factors of the main forest defoliator populations, it is proposed that specific chemical control be confined (1) to the years of progradation, (2) to the period when the young larvae are present and (3) only to certain limited surfaces considered strictly necessary.