The growth in weight of both the bursa of Fabricius and the spleen was investigated in different stocks of ducks between hatching and 154 days of age. The data were analysed by fitting the Janoschek growth curve and the allometric formula. Both organs showed rapid growth to their maximum dimensions and early points of inflection. The regression of the cloacal bursa started at about 12 weeks of age in Pekins and Mallards. In the Muscovy, the involution begins beyond the period of investigation. Allometrically, a biphasic relationship was found. Starting with positive allometry, the allometric exponent decreased to negative or isometric (Muscovies) values within the second week of age. If the organ weights are compared on the basis of these allometric relationships, Pekins had bursa weights that were about 30% higher than those in Mallards. For the spleen, there was no difference between wild and domesticated ducks.