In laboratory and field tests bee lice (Braula caeca Nitzsch) preferred younger (hive) bees (Apis meillfera) to older (worker) bees, and mated queens to virgin queens. Worker bees were preferred to drones at 25°C but in other laboratory and field tests no preference was demonstrated between the two cases. In laboratory tests there was a preference for young drones to old drones and for virgin and mated queens to young drones. Lice preferred virgin queens to old drones at 25° but no preference was observed at 34°. Mated queens were preferred to old drones, and virgin queens and mated queens over random-age workers. In nucleus colonies, one or more lice were observed on 24% of the queens between August and December. In field colonies, 62% of the queens examined from June through the rest of the season harboured lice; 58% of these lice were pale in colour indicating that they were less than 1 day old (callows). Among workers with lice, 98'6% had a single louse whereas 1'2% harboured 2 lice and 0'2% had 3; 4'2% of the lice were on drones. Levels of louse infestation in colonies decreased during May and June after oviposition by females. Populations then increased from July through the rest of the season, except for an unexplained drop during August.