The distribution of Arion lusitanicus Mabille and the damage caused by the slug were studied in1993-2009. A total of 38 sites were examined, characterised in terms of location, time of appearance and density of the slug, type and size of crops affected, and degree of damage to plants. At the start of the 1990's A. lusitanicus occurred only around Rzeszów and in the Sub-Carpathian subregion. In 1997 it started to appear in other regions of Poland. The rate of its spread was found to have increased significantly, particularly in the past few years. The slug mainly colonises urban areas (30 sites), and much less often – other areas (8 sites). The slug densities were the greatest in cultivations. The degree of damage done by the slug varied among plant species. Brassicas, lettuce, beetroot, carrots, beans, strawberries, numerous ornamental flower plant sand some weeds were the most damaged. The increase in the spreadof A. lusitanicus poses a serious threat to the biodiversity of native habitats and to garden-cultivated plants.