The experiments were carried out in Rzeszów, a region of the Arion lusitanicus mass occurrence, and also in Poznań. The studies were performed under field conditions, in isolators dug into the ground. It was found that the slug began egg laying by the end of the first decade of August. The period of egg laying, depending on meteorological conditions, lasted for 2-3.5 months. The eggs were laid in clutches, each containing from 5 to 193 eggs. Most of eggs laid in August hatched before winter. For the eggs laid in mid-August the period of hatching was shorter than those laid at the tum of August/September, and the percentage of hatched eggs was lower. Eggs deposited at later dates (in September-November) did not hatch before winter.