Trichodinids are commensal ectoparasitic protozoa, having direct life cycle and may become pathogenic hindering with respiration and feeding of fishes. The study aimed to determine the seasonal dynamics of Trichodina spp. infecting Oreochromis niloticus and their Histopathological effects. Microscopic examination of gills of 747 Tilapia at different localities of Behera province, Egypt was carried out all over one year extended from January to December 2019. The Histopathological examination of infected gills was performed. The results showed that, the prevalence of Trichodina spp. was 4.36%. The locality and weights of examined fish had no significance on the occurrence of Trichodina spp. The infection rate was affected significantly by the season. The highest infection was recorded in autumn whereas the lowest infection was in summer. Moreover, the measurements and morphological description of recovered Trichodina spp. were reported. Trichodina spp. appeared as multiple ring-shaped structures attached to the supporting gill structure and surrounded by few inflammatory cells and there was congestion, fusion, thickening and distortion of gills secondary lamellae.