In Iran, the rainbow trout (Onchorynchus mykiss) production has raised from 550 tonnes in 1990, to 1,332 tonnes in 1995 and reached a peak in 2009 with more than 62,000 tonnes of cultured fish. All provinces, particularly  those located in the main mountain ranges, Alborz and Zagros areas have  targeted the culture recently and the practice has undoubtedly seen great successes over the last decade. Although the potential of trout culture expansion is apparent, it is constrained by market demand and producer profitability. In Iran, trout  is widely sold and used fresh and some forms of value additions may also be observed. To know the production costs and their evolution is essential to the development and management of a trout farm. The characteristics of the trout farming industry are quite different, from extensive to highly intensive farms production of 3 tonnes per year to farms with more than 150 tonnes. A study of production, costs and profitability was carried out to help clarify trout production costs and their difference with location and farm size. Over the years 2005-2006, a total of 60 farms from the ten main provinces were randomly selected, classified and studied. The results of the survey showed that the various producer provinces have different cost  structures. Overall, feed with the highest level of variation accounted for 51% of total costs, followed by labour costs (11%) and larvae costs (11%). By 2005, total production costs averaged IRRial 117,800 per kg (US$ 1.98) in Iran. The costs sensitivity analysis shows feed costs is the most sensitive and 50% reduction in feed costs can decline more than 25% of the total fish cost per kilogram. On average, the larger farms were more efficient and had better conditions with averaged costs of IRRial 16,452 per kg (US$ 1.69).