Irradiation growth and creep are crucially dependent on the movement of dislocations under irradiation. However, despite being amongst the most commonly existing prior defects in polycrystals, the evolution of pre-existing dislocations and their impact on the formation of dislocation loops during irradiation have been relatively rarely investigated. In this study, the evolution of pre-existing dislocations and their impact on the formation of dislocation loops were investigated in magnesium by electron irradiation at different temperatures. The pre-existing dislocations have a profound influence on the loop size, density and Burgers vector of irradiation loops, and this influence is dependent on both the density of the pre-existing dislocations and irradiation temperature. Dislocation climb and glide were observed, but a significant change in pre-existing dislocation density was not seen. Display omitted