This study describes the infestation of raptor chicks with louse flies, based on frequent visits to nests with chicks of Buzzard Buteo buteo (n=79 visits, 1996-98), Sparrowhawk Accipiter nisus (n=30, 1996-98), Goshawk A. gentilis (n=8, 1996-98), European Honey-buzzard Pernis apivorus (n=8, 1996-98), Red Kite Milvus milvus (n=12, 1997-98, 2004) and Black Kite Milvus migrans (n=10, 1997-98, 2004). Nests (not the individual chicks) were coded as infested or not. Each chick was sexed (if possible) and measured (wing length and body mass). In Honey-buzzard (all visits in July) nor Black Kite (late May-mid June) louse flies were recorded. In Red Kite, only 1 out of 12 nests was infested, i.e. none of 10 nests in the 3rd decade of May and 1 of 2 nests in the first decade of June. Few Goshawk nests were infested with louse flies, i.e. none in late May (1 nest), once early June (n=6) and once late June (n=1). Mean number of nestlings was 1.5 in infested nests, and 3.17 in non-infested nests (notice small sample size). Sparrowhawk nests often held louse flies: 4 out of 9 nests in mid-June, 10 of 16 nests in late June, all 4 nests in early July and none in 1 nest in mid-July. Considerable variation in the annual infestation frequency was recorded: 1 out 7 nests in 1996, 10 out of 14 nests in 1997 and 7 out of 9 nests in 1998. The number of nestlings in infested nests averaged 4.06, against 4.25 in non-infested nests (difference not significant). Infested nests had a female-biased sex ratio of 34:40; non-infested nests had a slight preponderance of males (27:24). In the Buzzard, 8 out of 79 nests were found to have louse flies, mostly late in the breeding season: 0/2 nests early May, 0/6 nests mid-May, 0/32 nests late May, 0/24 nests early June, 7/13 nests mid-June, 1/2 nests late June. Infested nests averaged fewer chicks than non-infested nests, i.e. 1.25 and 1.58 respectively. Chicks on infested nests seemed to weigh slightly less than on nests without louse flies (Fig. I).