Adults of Balea fallax sampled monthly from the Roztocze Upland were dissected to examine the occurrence of uterine egg retention. Developing eggs were found in snails collected between May and August. In June more than 60% of the studied individuals were gravid, while in other months egg-retaining snails constituted less than 10% of the sample. The number of retained eggs ranged from 3 to 17 (mean 12.3) and was not related to the adult shell size. In June the majority of eggs contained shelled embryos (maximum shell size 1.1 whorl). The reproduction mode of B. fallax is compared with those of other ovoviviparous clausiliids of Central Europe.