An economic model of the costs of sheep flystrike has been modified to examine the effect of a given change in the risk of breech strike. Within each region (high rainfall, sheep/cereal and pastoral zones), the model sets the proportion of the sheep flock with a specified risk of breech strike, according to the proportion of untreated sheep struck in low, medium and high risk categories. The costs related to breech strike can be calculated according to the original model. The risk of breech strike can then be adjusted to any required level and the costs recalculated according to the adjusted risk. A 50% reduction in the risk of breech strike would decrease the number of sheep requiring preventive treatment to about one third previous levels, but only provide a small reduction in costs related to struck sheep, due to the large increase in the number of sheep no longer receiving preventive treatment. There would be only small reductions in costs related to crutching and mulesing sheep unless much larger reductions in strike risk occurred. The overall benefit of a 50% reduction in the risk of strike would be $0.23 to $0.27 per sheep. If this reduction in strike risk occurred across all regions this could provide a benefit to the Australian sheep industry of $15 million. If mulesing is not used in any of the sheep then the use of preventive treatment must increase so that 93% rather than only 60% receive chemical treatment, and reduction of the risk of breech strike by 50% provides benefits of $0.40 per sheep. Therefore, the estimated benefits would be greater than $0.27 per sheep in situations where the current level of risk is very high.