It would be of benefit to the dairy industry to have sires evaluated for the net economic value of their genetic potential for milk-production traits. Economic weights and selection indexes were derived for a variety of production circumstances, taking into account all returns and costs of dairy production and allowing for the existence of quotas. A single index designed for the average farmer in Ontario under a multiple-component pricing system was highly efficient (>98% of maximum gains) for most situations. Changes in pricing of milk caused the largest changes in the index. An index based on the current volume payment with fat correction would cause a 24% loss in efficiency of selection if protein eventually becomes twice the price of fat. A single index was proposed as highly efficient for farmers across Canada irrespective of their production circumstances, provided payment for milk is based on a multiple-component pricing system giving roughly equal value to fat and protein and low value to milk volume. Key words: Selection indexes, economics, milk production, sire selection