Measurements of 40‐kc acoustic damping in well‐annealed polycrystalline, 99.996% pure aluminum at temperatures between 4.2° and 273°K are compared with theories of dislocation damping by Granato and Lu¨cke and by Rogers. Two dislocation systems are indicated, both pinned by impurity atoms, with the average loop length of the one greater than that of the other. An amplitude‐dependent dynamic loss and an amplitude‐dependent breakaway loss for the long‐loop system, and the usual amplitude‐dependent breakaway loss for the short‐loop system are observed. The temperature dependence of various parameters important to the several types of damping is discussed and limits to the values of the types deduced for different temperatures. Electron‐lattice damping is observed at temperatures below 60°K and is found to be larger than theory predicts.