This paper presenis estimates of the indexes of internal returns to scale, external economies and mark-ups for 13 of the 14 manufacturing sectors of the NACE-CLIO R25 Spanish industries during the period 1964--1989. At the aggregate level, we find slightly increasing returns to scale and mark-ups well above one. However, at our level of aggregation, these results are biased by the presence of external economies that appear to be very significant. When these external economies are taken into account, the estimates of the indexes of internal returns to scale and mark-ups are greatly reduced in all cases.

Keywords: imperfect competition, mark-ups, industrial economics.