A haematological study was carried out with rainbow trouts experimentally infected with Yersinia ruckeri One hundred rainbow trouts (Oncorhynchus mykiss) at 192+/-0.24 g were examined. Yersinia ruckeri at a concentration of 3x10(6) Cfu/0.1 mi was intraperitoneally (i.p.) injected to 50 animals. The blood samples of infected and control groups were collected from caudal vein in 1, 2, 3, 4, 5, 7, 9, 11, 13 and 15 days after inoculation. Some haematological parameters: erythrocyte, leucocyte and thrombocyte counts; differential leucocyte types; haematocrit; mean cell volume (MCV), mean cell haemoglobin (MCH), mean cell haemoglobin concentrations (MCHC); haemoglobin, plasma total protein and albumin concentrations were determined.