PURPOSE: To obtain the title agent for preventing stress of fishes such as rainbow trout, salmon, eel, carp, tilapia, sea bream, etc., increasing resistance of fishes against microorganisms, viruses, etc., comprising a 4,9,10-trienesteroiod compound.

CONSTITUTION: A 4,9,11-trienesteroiod compound shown by the formula (R is 1-4C alkyl; X acyl derived from 1-18C carboxylic acid) is blended with feed for fishes comprising fish meal, protein, yeast, soybean powder, vitamin, etc., to give an agent for preventing stress of fishes effective against fishes of the family Salmonidae such as rainbow trout or salmon, Anguillidae such as eel, Cyprinidae such as carp, cycloid scale fish such as tilapia or the family Sparidae such as sea bream and for increasing resistance of fishes against microoganisms, viruses, bacteria and other pathogenic germs.