The anisotropy and magnetostriction constants of gallium‐substituted yttrium iron garnet single crystals of composition Y3Fe5−tGatO12with 0≤t≤1.5 have been investigated by means of ferrimagnetic resonance in the temperature range 4.2≤T≤500 K. From the concentration dependence atT=4.2 K the single‐ion contribution of the tetrahedral and octahedral anisotropy was found to beKd= −0.0019 cm−1andKa= −0.012 cm−1, respectively. In the ranget<1 of linear concentration dependence of the magnetostriction constants at T=4.2 K, a variation &Dgr;&lgr;100/t= −1.3×10−6and &Dgr;&lgr;111/t= +3.0×10−6was found.