A physiological study was made of the effects of age on the infectivity of eggs of the fowl nematode, Ascaridia galli and of dog and cat ascarids. The results from four experiments showed that the average number of A. galli per chick was markedly larger from the younger egg cultures than from the older cultures. Rather high infectivity of A. galli eggs existed until the cultures were 200 days old, after which a rapid decline in infectivity occurred. Hatching of eggs of A. galli, T. mystax., T. leonina was induced by an adaptation of Pitts’ method for Ascaris eggs. Measurements of the fat-containing areas in the bodies of the different aged larvae which were stained with Schlarch R demonstrated a diminution of fat (food reserve) with increase of age of larvae. Fat content was lost more rapidly in the younger than in the older larvae; however, the initial fall in fat content was greater than the fall in infectivity. The rate of fat diminution in larvae was hastened in maintaining egg cultures at higher temperatures, but retarded in maintaining the cultures at lower temperatures. Maintenance of embryonated egg cultures of the fowl nematode, A. galli, at optimum temperatures for periods of more than 200 days results in the loss of a large portion of the fat reserves and of most of the infectivity of the larvae.