The infection dynamics of Cyclops vicinus by the larvae of Bothriocephalus claviceps were studied experimentally. The mean number of procercoids/copepod rose to a plateau of approximately 14 as the density of eggs increased, whilst the number of copepods surviving decreased correspondingly. Copepods exposed to eggs of B. claviceps survived less well than controls, and those exposed to a higher density of eggs survived less well than those to a lower density. The mean number of parasites/copepod and the mortality of copepods were found to increase with the increasing time of exposure to eggs even when egg density was constant. These two parameters were also found to differ between gravid female copepods and unsexed young copepods. Young copepods suffered a higher parasite burden and thus a higher mortality, indicating that there are age or developmental-stage related differences in susceptibility of copepods to infection.