Condition factors calculated on the basis of weight and length are not a satisfactory index of fatness and a new index called a "fat factor" (F) is described. This is related to the specific gravity (G) of a fish by the equation F = G/(G − 1). Seasonal trends in F and a high positive correlation between F and percentage oil content of individual fish demonstrate its superiority over other condition factors as an index of fatness. A difference in either the specific gravity of, or ether-soluble material in, male and female gonads is reflected in the F values of male and female fish as a whole.