In the series La 2/3−x Tb x Ca 1/3 MnO 3 , it is known that the compositions are ferromagnetic for smaller values of x and show spin glass characteristics at larger values of x. Our studies on the magnetic properties of various compositions in the La 2/3−x Tb x Ca 1/3 MnO 3 series show that the cross over from ferromagnetic to spin glass region takes place above x≈1/8. Also, a low temperature anomaly at 30K, observed in the ac susceptibility curves, disappears for compositions above this critical value of x. A mixed phase region coexists in the narrow compositional range 0.1≤x≤0.125, indicating that the ferromagnetic to spin glass cross over is not abrupt.