We determined the earliest stage of postembryonic development at which primary and secondary sexual differentiations occur in the freshwater (“red claw”) crayfish Cherax quadricarinatus. .For this purpose, 797 males, 506 females and 456 intersex specimens within a weight range of 0.02-89.96 g were observed under stereoscopic microscope to determine the presence of the genital openings at the basis of the third (females) or fifth (males) pair of pereiopods. Animals presenting both pairs of genital openings were considered as intersex. A subsample of 106 males, 69 females and 59 intersex were dissected for the macroscopic characterization of gonad morphology. The development of the genital openings began approximately at 0.10 g, simultaneously with the differentiation of the reproductive system in females, males and intersex. Although the differentiation of the reproductive system started very early in the postembryonic development, the definitive form and colour of ovaries, oviducts, testes and vasa deferentia was acquired later. The differentiation of the appendix masculinabegan at 0.12-0.2 g and acquired the elongated shape of the adult at 1-2 g. The soft red patch characteristic of adult males started at 2.3 g of body weight in both chelipeds. | Determinamos el momento del desarrollo postembrionario en que se produce la diferenciación sexual primaria en la langosta Cherax quadricarinatus. Esta es evidenciada por la presencia de las gónadas y sus respectivos conductos. También determinamos la diferenciación sexual definida por la aparición de los caracteres sexuales secundarios. Se observaron 797 machos, 506 hembras y 456 individuos intersexos de 0.02 a 89.96 g (de criadero y laboratorio). Disecamos una submuestra de 106 machos, 69 hembras y 59 individuos intersexos para la caracterización macroscópica de la estructura gonadal. La diferenciación de los gonoporos se inicia aproximadamente a los 0.10 g, en sincronía con la diferenciación del sistema reproductor en machos, hembras e intersexos. La adquisición de la forma definitiva de ovario, oviducto, testículo y vaso deferente son posteriores. El appendix masculina inicia su diferenciación a los 0.12-0.2 g y adquiere los rasgos característicos del appendix de los adultos a partir de 1-2 g. La diferenciación de la mancha roja (red patch) ocurre a partir de los 2,3 g.