Barn owls produce large numbers of young, will nest in close proximity, are easily attracted to nest boxes, and occasionally form dense colonies. Their diet consists largely of various species of rodent pests. These characteristics suggest barn owls could contribute to pest control in agriculture. Studies have been conducted in Israel and Malaysia, but little quantitative research has documented their effectiveness. This study measured the effect of a population of barn owls on a rodent population in a 40-ha vineyard near Sacramento, California. In 2011, 11 of 20 boxes were occupied by breeding pairs, fledging 40 young. In 2012, 18 of 24 owl boxes were occupied, fledging 66 young; and in 2013, three of 24 boxes were occupied, fledging nine young. Nocturnal observations revealed the owls hunted the study area heavily. Monthly pocket gopher surveys using the mound-count method indicated that gophers declined on the vineyard with barn owl boxes relative to a control vineyard without barn owl boxes. Pellet analysis showed diet was composed mainly of Botta’s pocket gophers (70.4%) and California voles (26.2%). An infrared camera recorded 316 deliveries to a nest with three chicks (105.3 per chick) over the first eight weeks. Using these figures, and adding conservative estimates of adult consumption over the 165-day breeding season, and adult and fledgling consumption prior to dispersal, the total number of prey taken over the three breeding seasons was estimated to be 30,020 rodents. Cost comparison analysis showed an average cost of $8.11 per pocket gopher trapped versus $ 0.34 per rodent taken by barn owls.