Immersion of the adult female fresh water tropical teleost, Colisa fasciatus, in ice water at c. 2 degrees C for one minute immobilizes the fish. It, however, recovers within two minutes when returned to warm temperature. Hematological studies at recovery time intervals, viz., 3, 15 and 27 min and, later, at intervals of 48 min up to 363 min, and at 12 and 24 h, revealed significant changes in the abundance of leucocytes and thrombocytes: an increase in the number of leucocytes (leucocytosis) was evident from 15 through 123 min and at 267 min; normal leucocyte counts, were observed at 3, 171, 219, 315 and 363 min, and at 12 h and 24 h. Lymphocytes are the major leucocyte cell type in the blood. Thrombocytopenia occurred at 15 and 27 min, and thrombocyte counts were held at normal levels at other recovery time intervals. There was no change in the abundance of erythrocytes in response to the imposed stress. Shock susceptibility of the fish for variables measured was similar in experiments performed during six different months of one calendar year. In addition, no significant seasonal or diurnal changes were recorded in the abundance of circulating blood cells. The results reported here, for the first time on a tropical teleost, do not agree with previous results obtained from temperate teleosts by other workers.