The Examination of the soft part of the two bivalve species Venericardia antiquata (Linnaeus 1758) and Venus verrucosa (Linnaeus 1758), that occur together in northern coast of Tunisia, allowed us to discover for the first time the presence of morphological abnormalities affecting the foot of many individuals (annual rate of 31.6%). The presence of a developed byssus was also detected in some specimens of V. antiquata. A classification scale of this malformation, established depending on the degree of this anomaly, showed six initial types that evolve to form two or three feet, at the posterior and/or anterior sides of the animal. In order to determine the causes of this malformation, experiments of transplantation were carried out. Specimens of V. verrucosa collected from Zarzouna station were transplanted in Chaâra station which is characterized by low rate of malformations, low hydrodynamics and different sediment type and vice versa. Results revealed that foot malformations degree is highly correlated with both hydrodynamics and substrate type. This may be the main cause of foot malformations. The present study provides data on foot malformations in V. antiquata and V. verrucosa from the Tunisian coast that could be used as a starting point for future monitoring programs.