Gypsy moth (Lymantria dispar) larvae are highly polyphagous and can feed on more than 300 plant species, among which oak (Quercus sp.) is preferred. In Croatia outbreaks usually occur on its primary host, the pedunculate oak (Quercus robur), which makes up 201.739 ha and approximately 41.5 million m³, and therefore is considered the most important tree species in Croatian forestry. It is well known that Gypsy moth caterpillar defoliation causes compromised growth and increased mortality of oaks and lower resistance to other harmful biotic and abiotic factors. Almost all studies were made on oaks ; however, in the last outbreak period (2012-2014) a huge defoliation area was registered on beech (Fagus sylvatica) for the first time, and nothing is known about the consequences on this important forest species. In this study the performance of Gypsy moth caterpillars on the common beech foliage was investigated in the laboratory. About 150 caterpillars were weighed daily and pupae were sexed at the end of the experiment. Larval mortality was recorded every day and it was low with high pupation rate. Differences in the development between Gypsy moth larvae on oak and beech foliage were compared and discussed.