Abstract— Weigle (W)‐reactivation was demonstrated in Acinetobacter calcoaceticus for the UV‐irra‐diated lysogenic phage P78. The reactivation factor (survival of irradiated phage on irradiated bacteria/ survival on unirradiated bacteria) reached a maximum value of 20. This was obtained at UV‐doses giving phage and host survivals of about 5 times 10‐6 and 1 times 10‐1, respectively. Intracellular development of W‐reactivated P78 was followed by one‐step growth experiments. Conditions which allowed maximal W‐reactivation also extended the period of phage production and yielded a somewhat reduced burst size.