Forty-eight Hereford steers were used to determine the effects of Psoroptes avis (Hering) infestations on weight gain, feed conversion, and feed intake. Calves were randomly assigned to six pens with three pens per treatment. No calves were infected in the control pens, whereas two calves in each of the infested pens were initially exposed to P. ovis mites. The remaining calves in the infested pens became naturally infested by contact with the artificially infested calves. At the end of 94 days, infested calves had lower ( P < 0.01) daily gains and lower ( P < 0.05) gain-feed ratios than control calves. Feed dry-matter intake was similar in control and infested calves. Regression analysis suggested that P. ovis infestations covering less than 15% of body area did not affect daily weight gains but that infestations covering 30% of body area resulted in a ca. 30% reduction in daily weight gain.