Results of measurements of independent piezoelectric moduli d
11 and d
14 in Ca3NbGa3Si2O14, La3Ga5.5Ta0.5O14 and La3Ga5SiO14, promising single crystals of the calcium gallogermanate structural type, are presented. The moduli were measured with a triple-axis X-ray diffractometer under an external electric field which causes changes in the interplanar distances due to the reverse piezoelectric effect. The results of the X-ray diffractometry measurements agree fairly well (within less than 10%) with the results obtained by different methods.