Larger fish appear to be less frequently para- sitized than smaller ones potentially due to the death of young smaller fish or their lesser immunocompetence (Poulin, 2000). Trichodina spp. and Trichodinella spp. (Ciliophora: Trichodi- nidae) had the highest prevalence in this study, 9.7% and 13.6%, respectively. Their widespread occurrence and comparatively high prevalence might be explained by their capability to tem- porarily survive on several other non-fish hosts (Lom, 1995) and high organic load in Massaciuc- coli district’s waters