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Background. The investigation on different aspects of the reproductive biology is valuable information in the development of breeding technologies towards initiating conservation and other management measures of the indigenous fish stocks. The zig-zag eel, Mastacembelus armatus (Lacepède, 1800), is a commercially important fish species in the river Ganga of India. But, the data on the reproductive biology of this species is limited. Hence, this study was done to gather information on some aspects of the reproductive biology of M. armatus to address the gap in the current knowledge. Materials and methods. Some biological traits relating to the reproduction of M. armatus from the river Ganga were studied based on the monthly sampling for one year from April 2015 to March 2016. A total of 286 fish specimens were collected, comprising 167 males and 119 females. Weight–length relations for both sexes were estimated with the aid of regression analysis. The fecundity was calculated following the gravimetric method. The length at 50% maturity (L50) for the male and female were assessed employing maturity status data. The spawning season was predicted by employing gonadosomatic index and maturity status data. Results. Monthly sex ratio (F ÷ M) ranging between 0.22 ÷ 1 and 2.75 ÷ 1 with a mean of 0.71 ÷ 1 observed in the population was significantly different from 1 ÷ 1. Analysis of mean monthly gonadosomatic index (GSI) and the existence of matured (IV and V) and spent (VI) individuals, indicated year-round spawning with two breeding seasons. The first breeding season, in the river Ganga, occurred between February and May, and the other between July and November. The L50 of Mastacembelus armatus female and male were determined as 362.2 and 385.6 mm total length (TL), respectively. The reproductive potential in terms of absolute fecundity ranged between 2546 and 6058 with a mean of 3962 ova. Conclusion. The current investigation helps to report the size at maturity in the female and the male for the first time from Indian waters. Mastacembelus armatus represents a low fecundity, indicating that this species has a comparatively lower reproductive potential, making it more vulnerable to excessive fishing pressure. For the sustainable management of this species, it is necessary to utilize the resources wisely and avoid overfishing in the river Ganga.