The spinel ZnAl₂S₄ compound has been studied by electron spin resonance (ESR) technique. Paramagnetic Cr³⁺ and Mn²⁺ ions have been substituted for Al³⁺ and Zn²⁺ ions to investigate crystalline field symmetry and electronic structure as a function of the temperature between 10 and 300 K. Computer analysis of the ESR data indicated a high homogeneity and quality for the crystal. The magnetic electrons of Cr³⁺ ion are highly localized (98.6%) on Cr³⁺, while the unpaired electrons of Mn²⁺ are significantly delocalized (about 20%) to get covalent bonding with neighboring atoms (possibly Zn, S, Al). The ZnAl₂S₄ crystal is shown to be in the normal spinel structure without any significant distortion detectable even by a very high sensitive technique like ESR. The temperature does not affect these localization properties of the electrons either, indicating strong stability of the crystal symmetry.