Abstract: Variations in the density of the epizoic ciliate,Epistylis daphniae,on Moina macrocopa from a polluted stream were studied in 1990 in relation to the stage of life cycle of the host. Inall 13 samples, the numerical abundance of E. daphniae was not significantly correlated withaverage body length of the hosts or the percentage ofgravid hosts. Analysis of data from single samples showed that, in most cases,differences in the number of epizoites were highly signiflcant among the four size groups of Moina,but were not significant among the three smallgroups(mainly composed ofjuveniles).The number of epizoites on gravid Moina with embryos of later stages was significantly higher than that on Moina with embryos of earlierstages. It was concluded that the abundance of Epistylis attached on Moina was not determined by the surface area of the host,but was by the duration of the interval between twosuccessive ecdyses in the host.The epizoic ciliates on females with resting eggs and males ofMoina were also studied.