This study involved 344 Mullets, Mugil platanus Günther, 1880, captured in the estuarine region of Cananeia, São Paulo, Brazil (25º00'S, 47º55'W). The following data were recorded for each fish: total weight, total length, haemoglobin ratio (Hb), haematocrit (Ht), erythrocyte count (Er), differential leucocyte count (lymphocytes, monocytes, neutrophils, basophils and "other cells") and haematometric indices; MCV, MCH, and MCHC. The sex and stage of gonadal maturity (undifferentiated, juvenile, resting, maturing, mature, and spent) were determined by macroscopical inspection of the gonads. Parasitological inspections revealed the following infestations: Gills - Trichodinidae, Monogenea, Copepoda, and Hirudinea. Liver: cysts of Digenea. Intestine: Nematoda, Digenea and Acanthocephala. Blood: Trypanosoma sp. and Haemogregarina sp. Analysis of variance was done to evaluate how far blood and gill parasitism had any effect on total length, total weight, the biological condition factor or the measured blood parameters. Significant differences were found for the haematocrit (infested versus noninfested by Trichodinidae) and for MCHC (infested versus noninfested by Trichodinidae and Copepoda). No differences were found with regard to leucocytes.