Helminthology and haematology parameters have been recognized as the important tools for assessing the fish health. Measurement of blood parameters has been used for many years as a tool for monitoring the health of fish. The present study investigated the effects of a natural infestation of Siganus rivulatus, lethrinus harak and Gerras oyena with internal parasites on blood parameters such as red blood cell (RBCs) and white blood cells counts (WBCs) and hemoglobin level were estimated as well as plasma glucose and serum activities of liver and kidneys function tests. Our study was conducted on 275 marine fish 100 of Siganus rivulatus, 100 of lethrinus harak and 75 of Gerras oyena collected from water of Safaga city at Red Sea governorate for examination of internal parasitic infestation. Parasitized fish showed decreases RBCs level (1.41 x 106 /μL), low hematocrit (17.82%) and increases in WBCs (18.37 x103 μL) and increases in plasma glucose levels (68.4 mg/dl), while decreased in serum total proteins (2.81 g/dl). AST activities in serum (95.1 U/L), ALT activities in serum (81.65 U/L) and Creatinine value (1.10 mg/dl) all increases compared with values in healthy non infected fish due to increase of helminthes infection. Decrease in serum urea (24.25 mg/dl) level was also found. The result revealed that fish were apparently healthy and no pathgnomic signs of infested fish except excessive mucous secretion. In case of P. M. examination of infested fish liver there was paleness with areas of hemorrhage and somewhat Congestion with peticeal hemorrhage, inflammation of Intestine. It was 46.0% of examined fish infested with Nematodes spp. were recorded. The highest percentage of infestation with Nematode spp. was more than150 g. and 25 cm. in case of Siganus rivulatus, 14. % in Lethrinus harak. and 60% in Gerras oyena