The ovary structure of the hermit crab Diogenes pugilator (Crustacea: Decapoda), is described in the different phases using electron microscopy. Sexual maturity was reached at a very small size; females from 0.74 mm shield length (SL) could be found in vitellogenesis stage and even 0.6 mm SL ovigerous females exist. The biometric study of the ovigerous females by independent size-class (non-accumulative) shows that 1.3 mm SL is the size at which 50% of females are ovigerous; this value is increased a little (1.7–1.8 mm) if the population of females, or ovigerous females, is analysed from an accumulative point of view. In addition, an optimal reproductive size (between 2.1 and 2.3 mm SL) and a reproductive decrease in the maximum size-classes have been detected. The relation between brood size (measured through egg number) and the hermit crab size follows a potential function. The number of eggs by brood varies in a wide range (9–2838), with a mean diameter of 0.35±0.03 mm. No relationship between egg size and shield length has been found.