The present investigation is concerned with the protozoan parasites of the freshwater crayfish, Procambarus
clarkii, wnidv was accV&erxtatty introduced to the River Nile in Egypt during the 1980s. A total of 120 specimens were
collected from different water localities in Giza, Cairo and Qualyubia governorates, brought to the laboratory and
microscopically examined. Three protozoan parasites were detected either in the gills or in the connective tissue underneath the carapace. The most common parasite was the ciliate, Epistylis cambari, which was found on the gills of 36 out of the 120 specimens examined (30%). Another parasitic ciliate of the genus Tracheluis was also found on the gills of only 3 out of the 120 specimens examined (2.5%). Cysts of a third parasite, Psorospurmium sp. were detected in smears of the connective tissue underneath the carapace of a single specimen. The morphology of the three recorded parasites is described by light microscopy and the SEM structure of Epistylis cambari and its pathological effect on
the gills were also studied. The taxonomic position of Psorospurmium sp., which is recorded for the first time in Egypt
in the present investigation, was discussed.