Honey bee colonies under a 765-kV power line (electric field [E-field[ 7 kV/m) showed effects, with hive height and weather-influenced hive current distribution the controlling factors. Hive current was a mean of 59 μA(= 1 m hive height) or 85 μA (1.5 m). Shielded controls under the line had E-fields and hive currents 150- to 300- fold and 100-fold lower, respectively, than exposed hives. Groups were exposed or shielded 8 or 16 weeks, with treatment reversal for some at week 8. A 16-week exposure did not affect percent honey moisture or teneral adult weight. When first exposed, colonies exhibited pronounced but transient elevations in temperature. Capped brood was normal in exposed 1-m hives but declined (P<0.01) in 1.5-m hives by week 4, associated with higher incidence of queen loss and queen cells and colony failure. Weight gain was depressed in all exposed hives after 2 weeks, being severe in 1.5-m hives (P<0.001 at week 5) and intermediate in 1-m hives (P = 0.05 at week 8). Effects were more pronounced the first 8 weeks than the last 8. Painted hive interiors reduced bio-effects, presumably by reducing internal current flow. Only exposed colonies propolized hive entrances, but amount and time of onset were not dose related. Treatment reversal at midseason resulted in reversal of colony behavior, manifested more with hive weight than brood. Overwinter survival of exposed groups was ≤29% compared with ≥71% among shielded ones.