Two copepod groups-Acartia bifilosa adults and nauplii-were examined for the effect of preservation with 4% formalin solution on their total body length. The length of the same individual was measured before, immediately, one week, and two months after formalin fixation. There was no statistically significant difference in live and preserved copepod body length at any time of measurement either in the case of adults or nauplii. Therefore it is suggested that the body length of copepods drawn from preserved samples can be used as an unbiased measure for the calculation of copepod biomass from the length-weight relationship.