ABSTRACT. The population biology and size at sexual maturity of Uca (Uca) maracoani were studied for the first time in a semi-arid tropical estuary of northeastern Brazil. A catch-per-unit-effort (CPUE) technique was used to sample the crabs, on the surface and inside burrows, for 60-min sampling periods by one person on a monthly basis during spring low tide periods from June 2013 to June 2014 in an estuary area of Baixa Grande Beach. A total of 406 crabs were obtained, of which 317 were males, 87 non-ovigerous females, and two ovigerous females. Males were larger than non-ovigerous females. The U. (U.) maracoani population presented unimodal size frequency distribution, which may reflect a continuous recruitment, with no disruption of classes and constant mortality rates, indicating a stable population. The overall sex ratio (3.5 males: 1 female) differed significantly from the expected 1:1 proportion. The major cheliped was on the right side in 45.1% of the males, as reported for other fiddler crabs populations. A total of 294 males and 89 females were used in the allometric study. The specimens were measured at carapace width (CW), the major propodus length (MPL) of males, and abdomen width (AW) of females. In males, the relationship between CW and MPL was 22.3 mm CW, which is considered the functional value at maturity. In females, the size at sexual maturity was 19.3 mm CW based on the relationship between CW and AW. The CPUE method is discussed as one of the reasons of the high sex ratio skewed towards males. This is the first account regarding population structure of U. (U.) maracoani in semi-arid tropical areas. Keywords: Uca (Uca) maracoani, population structure, sex ratio, sexual maturity, CPUE technique.Estructura poblacional y madurez sexual del cangrejo Uca (Uca) maracoani (Latreille, 1802) (Crustacea: Decapoda: Ocypodidae) en un manglar semiárido tropical en el noreste de Brasil RESUMEN. La estructura poblacional y la madurez sexual morfométrica del cangrejo Uca (Uca) maracoani fueron estudiadas en un manglar tropical en el noreste de Brasil. Los cangrejos fueron colectados mensualmente durante los periodos de marea baja de junio 2013 a junio 2014. Se utilizó la técnica de captura por unidad de esfuerzo (CPUE) con períodos de una hora de captura realizada por una persona. Se obtuvo un total de 406 cangrejos: 317 machos, 87 hembras no-ovígeras y dos hembras ovígeras. El tamaño de los machos fue significativamente mayor que el de las hembras no-ovígeras. La población mostró una distribución unimodal de frecuencias de tamaños, lo que puede reflejar un reclutamiento continuo, sin interrupción de las clases y las tasas de mortalidad constantes, lo que indica una población estable. La proporción sexual (3.5 machos: 1 hembras) se alejó significativamente de la proporción esperada 1:1. El quelípodo mayor estaba en el lado derecho en el 45,1% de los machos, según lo informado por otras poblaciones.