Reproduction of Arion lusitanicus Mabille was studied in the field, in a region of the slug’s mass occurrence, and in laboratory, at temperatures of 10, 15, 20 and 25 °C. Places of egg-laying, fecundity and development rate were determined. In the field, A. lusitanicus mated in the third decade of July and started laying eggs early in August. The eggs were laid in clutches on or in the soil. In horticultural and agricultural crops the slug laid the most numerous eggs on field edges and baulks. An individual egg production was ca. 400, from 5 to 190 eggs per clutch. The first juveniles hatched at the end of October. In laboratory a close correlation was found between the duration of incubation period and the percentage of hatched eggs. The incubation period was the shortest at 20 °C, the mean percentage of hatched eggs being the highest at 15 °C. No eggs hatched at 25 °C.