A method is described for estimating in situ biomass production rates of copepods. The youngest naupliar stages are isolated by filtration through two different size sieves, and growth is measured after laboratory incubation by filtering through a series of eight filters. Since the filters appear to be relatively efficient at separating copepods of different sizes, copepod size can be estimated from the pore size of the filters alone, resulting in a very rapid procedure for calculating biomass. This method allows experimental manipulation of environmental conditions, permitting for example, an estimation of the relative importance of temperature and food abundance in the seasonal growth cycles of these organisms. It was concluded that, in the Burlington Canal, temperature exerts a greater influence on growth rate than does food concentration, although adaptation to food levels may be expressed indirectly, due to a similarity in the seasonal cycles for temperature and food. The method is also useful in predicting the potential impact of toxic substances on copepod production rates under natural conditions. Key words: copepods, zooplankton, biomass production, temperature, food concentration