The effect of four oak species: Turkey oak, Hungarian oak, sessile oak and English oak on the number of moultings and head capsule width of the gypsy moth caterpillars was researched. Host plants have a statistically significant effect on the number of both male and female larval instars. The highest number of larval instars was recorded for caterpillars fed on sessile oak leaves, and the lowest number was recorded for those fed on Turkey oak leaves. Females reared on Hungarian oak and English oak leaves had a normal number of larval instars. Except for 1st instar caterpillars, all other larval instars showed a statistically significant host plant effect on the head capsule width. The widest head capsules were found on caterpillars fed on Turkey oak leaves, and those fed on sessile oak leaves had the narrowest head capsules. The study data on head capsule widths can be practically applied in the determination of the optimal ratio of larval instars in the gypsy moth population, i.e. in the determination of terms for aerial spraying actions.