Twelve heronries were investigated in the Po plain (northwestern Italy) from 1974 to 1984. A total of 219 blood films was obtained from chicks and young of the night heronNycticorax nycticoraxand the little egretEgretta garzetta,24.6% of which harbouredLeucocytozoonparasites. Infection prevalence varied between heronries, mainly due to variation in simulid abundance. The abundance of vectors was affected by the agricultural practices for rice cultivation: irrigation canals and ditches were periodically drained and cleaned out with the consequent destruction of the aquatic vegetation in which simulid larvae and pupae settle. Discrimination of heron colonies (number of cases) where infection was detected from those where infection was not detected was mainly by simulid abundance, and secondly by colony population size. From 1974–1979 to 1980–1984 the increase in infection prevalence seems to be associated either with the increase in the number of breeding herons or the decrease in the number and surface area of heronries.Leucocytozoonparasites hardly seem to be pathogenic because they did not affect breeding success and did not reduce the fitness of the hosts.