In this work the ferromagnetic state in a ferromagnetic insulating manganite viz. Pr0.8 Sr0.2 Mn O3 was analyzed. The temperature variation of ac susceptibility is seen to show two transitions with a high temperature transition occurring at 158 K and a low temperature cusp (Tf) around 90 K. The high temperature transition (TC) was found to be independent of frequency while the low temperature transition shifts with frequency indicating a frustrated magnetic state. The relaxation time observed from the critical slowing down spin analysis indicates that this state is that of a cluster glass. Temperature variation of resistivity shows an insulating behavior with two distinct humps corresponding to TC and Tf. Temperature variation of thermopower exhibits a peak at TC and decreases below TC which is attributed to the sudden release of holes from traps, similar to a ferromagnetic metallic manganite. Magnetoresistance is seen to show a change in curvature upon cooling the sample across TC which in conjunction with the peak in resistivity and thermopower suggests that this transition is mediated by a double exchange mechanism. The frustrated state is seen to arise because of a competition between double exchange and superexchange mechanisms. The effect of double exchange mechanism is seen to persist all the way down to 5 K as seen from the magnetoresistance analysis. These results clearly suggest that double exchange still plays an important role in dictating the magnetic and electrical properties in ferromagnetic insulating manganites. © 2006 American Institute of Physics.