Aegla lata Bond-Buckup & Buckup, 1994 is a critically endangered species, extinct in its type-locality. Its currently known geographical distribution is restricted to small streams of the lower Tibagi River Basin (Paraná state, southern Brazil), where it is vulnerable to the environmental degradation that takes place in these watersheds. Scientific knowledge on the species is limited to the original description and a few occurrence records. No information about its biology is available. Aiming to fill this gap, we investigated the relative growth, sexual maturity, sexual dimorphism, and the reproductive and recruitment periods of A. lata. Monthly sampling occurred from May 2014 through June 2015 in the Bule stream in the southern limit of Mata dos Godoy State Park, Londrina, Paraná state, Brazil (23°27ʹ20.86″S, 51°16ʹ32.44″W). The analysis of allometric growth was used to differentiate juveniles and adults. According to the development stage of the chelae, two sequential groups of adult males were recognized (morphotypes I and II). The estimated average cephalothorax length (CL) at the onset of morphometric maturity of males and females are similar, suggesting puberty molt takes place at nearly the same size in both sexes. Males and females are heterochelous, with a clear predominance of the left chela as the most developed of the pair. Both chelae are larger in adult males than in adult females of similar CL. The reproductive period is markedly seasonal, extending from the austral autumn to winter (May to August), as well as those recorded in other species of Aegla from similar latitudes. A single annual recruitment pulse was detected from October 2014 to February 2015.