In 2 field tests on a commercial poultry ranch, different concentrations of a pyrethroid, fenvalerate, were compared against the standard carbaryl by hand and power sprays to layer hens for control of Ornithonyssus sylviarum (Canestrini and Fanzago). Experimental hand spray concentrations of 0.05 and 0.025% fenvalerate and 0.5% carbaryl produced effective mite control for 2 months during which time the treated hens were challenged with mites at 3, 6, and 8 wk posttreatment. In a 2nd test, commercial power sprays of 0.025 and 0.0125% fenvalerate produced effective mite control for 53 days. No adverse effects, loss in egg production, or abnormal mortality occurred in hens sprayed with the experimental formulation of fenvalerate.