In urban areas the turnover of species is fairly rapid, species of natural habitats become eliminated and invasion of alien species takes place. An example of these process was investigated in Łódź, Poland. The present mollusc fauna of a city park was compared with the results obtained 40 years ago. The estimated turnover index for terrestrial gastropods was 0.29 and for molluscs of park ponds – 0.4. In the recent inventory 57 species were recorded, compared to the 48 in the 1950s. Some stenoecious gastropods, e.g. Discus ruderatus, Ruthenica filograna, Malacolimax tenellus, Nesovitrea petronella were not re-found. Species recorded for the first time included synanthropic Oxychilus draparnaudi, Helix pomatia, Cepaea nemoralis, Arion rufus, Limax maximus and open-country snails, e.g. Cochlicopa lubricella and Vallonia excentrica. Considerable changes were observed in the composition of the malacofauna of park ponds. The following species were found for the first time: Viviparus contectus, Potamopyrgus antipodarum, Hippeutis complanatus and Anodonta cygnaea. The newly recorded species were probably brought with soil, plants, stocking fish, could be transported by birds or introduced on purpose. The changes in the fauna result also from intense penetration by humans, decrease in the ground water level, emptying and cleaning of the ponds and changes in the water supply.