In this paper, we report on the spectral dependence of Δn=n _o-n_e for CdGa₂S₄ single crystals for shorter and longer wavelengths than the isotropic wavelength λ₀=485.7 nm (300 K). It was established that Δn is positive at λ>λ₀ and it is negative in the spectral range λ<λ₀. The isotropic wavelength λ₀ exhibits blue spectral shift with temperature decreasing. The ground and excited states of three excitonic series A, B and C with binding energies of 53 meV, 52 meV and 46 meV, respectively, were found out at 10 K. The effective masses of electrons for k=0 were derived from the calculation of excitonic spectra: m_c^â̂¥(Eâ̂ ¥c)=0.21m₀ and m_c ^⊥(E⊥c)=0.19m₀. The holes masses are equal to 0.59m₀ and 0.71m₀ for Eâ̂¥c and E⊥c, respectively. The value of valence bands splitting, V₁-V₂, by crystalline field equals 24 meV, and V₂-V₃ splitting due to the spin-orbital interaction equals to 130 meV. The optical functions n, k, ε₁ and ε₂ for E⊥c and Eâ̂¥c polarizations were calculated by means of Kramers-Kronig analyses in the energy interval 3-6 eV. The evidenced features are discussed taking into account the results of new theoretical calculations of CdGa₂S₄ band structure.