Simple stoichiometric models based on the principle that limiting elements are used with high efficiency have been unable to capture the apparently constant and low nitrogen gross growth efficiency that characterizes egg production in marine copepods. A new model of egg production is presented based on Dynamic Energy Budget theory. The model splits substrates between nitrogenous and non-nitrogenous compounds, distinguishes between structural and reserve components of animal biomass, and requires that assimilated substrates are utilized to meet maintenance costs prior to allocation for egg production. Carbon and nitrogen gross growth efficiencies of egg production by Acartia in response to food C:N are predicted realistically. Production tends to be maximized when consuming N-rich food, indicating a general N-limitation, the benefits being greatest at high intake rates. However, food carbon also plays a role in regulating egg production by supporting protein-sparing biochemical pathways. The model indicates that zooplankton are unable to utilize dietary N efficiently for egg production, even when it is scarce in the diet, because of the N demands for maintenance by the adult. The work emphasizes the need to consider the C and N requirements for maintenance in order to understand dietary constraints to mesozooplankton production.