Irradiation of silicon crystals by high-energy electrons, f -rays, neutrons, o r ions is known to lead t o the generation of radiation defects which cause the appearance of a number of electron-vibrational bands in luminescence spectra /1 to 4/. The presence of narrow zero-phonon emission lines in these spectra permits an effective use of uniaxial s t r e s s effects for determining the symmet r y of the corresponding luminescence centres /5/. Knowledge of this important microscopic characteristic is, in most cases, a decisive factor for the choice of geometrical and physical models and ultimately for the establishment of the irradiation-induced defect nature.The luminescence band with the zero-phonon line at 1.0186 eV has been observed in pulled and float-zone silicon crystals after irradiation by neutrons /3/ and various ions /2, 4/ and annealing in the temperature range 50 to 350 OC. Previously /2/, the centres responsible for this band, considering the apparent impurity independence, annealing properties, and comparison with the E P R data, have been tentatively identified with the five vacancy cluster defects (Si-P1 centre),which have Clh(= Cs) symmetry /6/.In this note we report the results of uniaxial stress studies on the 1.0186 eV emission line performed with the purpose to determine the symmetry of the corresponding centres. F r o m the analysis of uniaxial s t r e s s splitting of the line and the relative intensity of the stress-split components under a polarized light, it is found that the centres have a trigonal symmetry with a circular magnetic dipole oscillator in the plane normal to the (111) centre axis. The preliminary data of these investigations have been reported in /?/.