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.<br/> 67 species were observed, of which the most frequently noted phytophagous lepidopters were: (1) Dryobotodes eremita Fabr. and D. Monochroma Esp., - with latent fluctuation; (2) Tortrix viridana L., Ortollosia cruda Schiff. and Spudaea ruticilla Esp., - with temporary fluctuation; and (3) Phalera bucephala L., Malacosola neustria L. and Porthetria dispar (L.), - with periodic fluctuation. P. dispar (L.) and M. Neustria L. fluctuations occurred in cycles of approximately 9 years. Severe defoliation was observed in 1973 and 1974, Malacosoma reaching maximum population density a year earlier than Porthetria.<br/> Numerous natural mortality factors affected the population fluctuation of Porthetria. 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 parasites Ooencyrtus kuwanai (How.) (Hym., Encyrtidae) and Anastatus disparis Ruschka (Hym. Eupelmidae) and the predators Dermestes lardarius L. (Col. Dermestidae) and Haplocnemus jejunus Kiensenw. (Col. Melyridae).<br/> 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 were Sturmia scutellata R.D., Exorista segregata Rond., Exorista larvarum L. (Dipt. Tachinidae) and Sarcophagidae of Agria and Sarcophaga genera. The predator Calasoma sychophanta L. (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 the Porthetria chrysalides: Pales pavida Meig. (Dipt. Tachinidae), Muscina stabulans Fall. (Dipt. Muscidae), Pimpla instigator F., Theronia atalantae Poda (Hym., Ichneumonidae) and Brachymeria intermedia (Nees) (Hym., Chalcididae). Porthetria population fluctuations were also affected by ambiental factors and by a reduction in fecundity when population densities were at their highest.<br/> Malacosoma neustria L., apart from being subject to the same parasites and pathogenetic agents as Porthetria, was particularly influenced by Pales pavida Meig. and Scambus malacosomae Seyrig (a hymenopter parasite specific to Malacosoma larvae in cocoon).<br/> 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.<br/> 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.<br/> 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.