Seasonal variations and the effects of host size and age on parasite prevalence and abundance were examined in a wild population of common carp (Cyprinus carpio) from Iznik Lake in Turkey. Twelve monthly samples were collected which revealed one species of monogenean, Dactylogyrus extensus, and two species of cestode, Caryophyllaeus laticeps and Bothriocephalus acheilognathi. Abundance of D. extensus varied - highly significantly with month increasing from April to peak in May-July, followed by a sharp decrease thereafter. These changes followed the water temperature pattern and a significant positive correlation was found between parasite abundance and water temperature. Significant differences were also revealed in the abundance of C laticeps indicating a temperature-dependent response. Infection was not detected from July to October, most fish acquired parasites during the period December-February, and the parasite population showed a decline in spring until July when the infection disappeared. Of the three species only D. extensus showed a clear tendency of increase in infrapopulation size with host size. The distribution of infrapopulations of D. extensus varied highly significantly with the size class of fish, peaking in large size classes and the correlation of infrapopulation size and fish length was highly significant. A substantial peak in the abundance levels of this monogenean in mature fish was detected in spring and summer, related to spawning.