Stickleback (Gasterosteus aculeatus L.) living in Spain occurs in rivers flowing to the Mediterranean and Cantabrian Seas and to the Atlantic Ocean. Taxonomically studied populations from North-Eastern Spain (La Tordera and Muga rivers drainage basins) belong to low-plated morph (leiura) as the rest of all Spanish populations. According to meristic characters (number of plates, number of rays in dorsal and anal fins, etc.), these populations are similar to other “leiura” specimens from Alger, Italy and France, however when comparing these populations with “trachurus” specimens from Poland and European Soviet Union, noticeable differences appear in body shape: Spanish specimens are characterised by a shorter thorax and first dorsal and ventral spines. Also markedly bigger dorsal, anal and pectoral fins are evident.