Interstitial dislocation loops with Burgers vector of type are observed in α-iron irradiated by neutrons or heavy ions, and their population increases with increasing temperature. Their effect on motion of a edge dislocation was reported earlier 1. Results are presented of a molecular dynamics study of interactions between a screw dislocation and loops in iron at temperature in the range 100 to 600 K. A variety of reaction mechanisms and outcomes are observed and classified in terms of the resulting dislocation configuration and the maximum stress required for the dislocation to break away. The highest obstacle resistance arises when the loop is absorbed to form a helical turn on the screw dislocation line, for the dislocation cannot glide away until the turn closes and a loop is released with the same Burgers vector as the line. Other than one situation found, in which no dislocation–loop reaction occurs, the weakest obstacle strength is found when the original loop is restored at the end of the reaction. The important role of the cross-slip and the influence of model boundary conditions are emphasised and demonstrated by examples.