An equation is presented to facilitate estimation of the production of the cosmopolitan cyclopoid copepod Oithona similis. The egg hatching rate was studied from Arctic, subarctic and temperate waters covering a temperature interval from -1 to 20.5degreesC. Within this temperature range the hatching rate (HR) increased from 0.03 to 0.42 d(-1). Results from all experiments were fitted to a function HR (% d(-1)) = 4.2176 + 1.7545T (r(2) = 0.98; p < 0.0001), where T = temperature. When combined with site-specific information on temperature, egg:female ratios and the carbon content of females and eggs, secondary production of this ubiquitous species can be readily estimated.