The specimens of the freshwater snail Physella acuta collected from Southeastern part of Bulgaria during the cold period as a whole were with mean shell height of 5.7 mm. During spring and summer it was more than twice lower, 2.3 mm. The ratio of the size groups was more equally spread during cold seasons rather than in warm ones. The variation index during cold seasons is about eight times higher than in the warm ones (Var = 3.96 and 0.58, respectively). On both plant species the gastropods had similar mean shell heights. For C. demersum it was 4.8 mm (min-max = 2.5-8.9 mm), and for E. canadensis 7.2 mm (min-max = 3.6-10.0 mm).