Optical activity along the optical axis of four piezoelectric crystals Sr3TaGa3Si2O14 (STGS), Sr3NbGa3Si2O14 (SNGS), Ca3TaGa3Si2O14 (CTGS) and Ca3NbGa3Si2O14 (CNGS) with ordered langasite structure were studied via a convenient method. All the crystals investigated show unusually large values of the specific rotation. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)