Production [P], biomass [B] and P/.hivin.B ratio was calculated for a single species population of P. hammoniensis (Michaelsen) in the profundal of Lake Esrom [Denmark]. Two methods were compared. The 1st was the Allen curve method in which numbers were plotted against mean weight for a cohort hatching in 1956. The 2nd involved using estimates of instantaneous growth rates on a daily basis extrapolated to annual rates by calculation of the number of degree-days available for growth. Annual rates could then be calculated using the formula P = g B. The 2 methods gave almost identical results. Yearly total production (P) during periods of growth was on an average 998 mg, mean biomass (.hivin.B) 1222 mg dry wt m-2 yr-1 and P/.hivin.B 0.82. Between year changes were small. Production of cocoons amounted to 4-30% (average 15%) of total annual production. Negative production during the stratification period was about 16% of total annual production during periods of growth, and P/.hivin.B was also lowered by 16% to 0.68 P and .hivin.B increased, but P/.hivin.B decreased as the 1956 population grew older during their 4 yr life span. This very abundant population made up the majority of total production when 3 yr old.