The latest outbreak of the gypsy moth Lymantria dispar L. gave rise to investigations on the gypsy moth dynamics in southern Switzerland. Densities and development of different gypsy moth instars were monitored. Additional information was obtained by semi-quantitative observations of gypsy moth antagonists. The population increase commenced in 1991 near Bellinzona and resulted in complete defoliation at various places in 1992 and 1993. In the first year of the outbreak regulating factors such as hunger, suboptimal host plants, predators, parasitoids, and a virus disease (nuclear polyhedrosis) already began to affect population development. In the second year of defoliation the populations collapsed due to the polyhedrosis. There were no places with two consecutive years of complete defoliation. The survival of the affected stands of chestnut trees was not jeopardized at any time.