Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) ; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) ; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) ; Processo FAPESP: 94/4878-4 ; Processo FAPESP: 98/3136-4 ; Processo FAPESP: 04/15194-6 ; Studies on the post-larval morphology and growth of crabs can allow correct identification of their early life stages, supporting studies on phylogenetic relationships and ecological aspects. Nevertheless, little is known about the juvenile morphology of ocypodoid crabs worldwide. Uca cumulanta Crane, 1943 is a fiddler crab commonly found in the intertidal zone of the tropical and subtropical western Atlantic. This study describes the morphology of the first juvenile stage of U. cumulanta, its absolute and relative growth, and the appearance and development of its secondary sexual characters. The antennules of U. cumulanta show morphological peculiarities and could probably be used as a distinguishing feature for crabs of this genus. The maxillule and the maxilla have sets of setae forming specialised structures for sorting particles from the sediment. The absolute growth pattern differed statistically between sexes. The study of relative growth revealed differences in the relationships cheliped propodus length (CPL), cheliped propodus height (CPH), and abdomen width (AW) vs. carapace length (CL). These differences between sexes showed that males diverge from females (in the chelipeds) from 3.04 +/- 0.11 mm (VI juvenile stage), and females diverge from males (in the abdomen) from 3.84 +/- 0.13 mm (VII juvenile stage). The pleopods, rudimentary in the first stage, disappear in the second stage, and then arise in different numbers for each sex from the third juvenile stage on. The spoon-tipped setae found on the second maxilliped, which are used to sort food particles, are evident only from the fourth juvenile stage and increase linearly with carapace growth. The absence of these setae may be the reason why juveniles settle on organic-rich substrates where they can obtain food.