Background: Mandibular organ (MO) in decapods is suggested to play regulatory role in reproduction, in few species; however, MO is considered to control growth. The present study addresses this question by an ultrastructural study on the MO of the field crab, Paratelphusa sp. Our sampling for consecutive years (2008 to 2012) revealed that Paratelphusa sp. devotes July to October for reproduction, judged by the occurrence of growing ovaries and the berried females. From November to the succeeding June, the females are in a state of reproductive arrest (non-reproductive period); ovaries during this season would appear as white bands with no signs of yolk deposition. Results: Morphologically, MO of Paratelphusa sp. is positioned posterior to the mandibles and is in close apposition with the distal end of the mandibular apodeme. MO of Paratelphusa sp. exhibited significant levels (t = 8.097, P < 0.0001, N = 10) of season-dependent size variations. Our electron microscopic observations reveal that the MO is highly secretory during the reproductive period, evidenced by the occurrence of sacculated Golgi bodies having dense inclusions, several mitochondria with tubular cristae, and extensive networks of SER and rough endoplasmic reticulum (RER). During the non-reproductive period, however, the MO is least active; RER, the mitochondria, and the Golgi are only sparsely seen. Interestingly, the plasma membrane exhibits a highly convoluted appearance all the way through the non-reproductive period. Conclusions: The present study reveals that the secretory activity of MO of Paratelphusa sp. is entrained with reproductive activity. The existence of a high correlation between MO secretory activity and ovarian growth implicates the former’s role in reproduction. © 2014 Sarika et al.; licensee Springer.