The study of population dynamics in a fishery and the regulation of a fishery require that fishing effort be measures. This paper explores the use of cross section production functions to estiamte the fishing power of individual vessels. The problems addressed int he study are: The proper measurement of output; the measurement of fishing time; important vessel characteristics; crew size; the effect of location, and, the measurement of technological change. Regression analysis upon data from the North Atlantic groundfish fishery and the tropical tuna seine fishery yielded highly significant results. Many of the hypothesized relationships are measureable and stable with relatively small erros. Briefly the tests indicate that: There are better measures of putput than total pounds; fishing time is measures better using days absent rather than days fishing; that the use of more vessel characteristics improve explanatory power; that crew size can be an important variable; that the effects of location can be measured; and, that technological change can be measured. The production functions measured can then be used to develop indices of fishing power that can be applied to each vessel in a fleet. The indices can then be multiplied by fishing time and aggregated into an index of total effort. The ramifications of the study are many. It gives a simple way to biuld effort indices for many fleets and points the way to rationalized data collection.