Fry production in Nile tilapia, Oreochromis niloticus is typically achieved by mass spawning of males and females stocked in large hapas suspended in ponds. In such spawning aggregations, territorial behavior and reproductive competition among males may lead to a large variance in reproductive success among individual males. We analyzed parentage and quantified male reproductive success of O. niloticus under commercial hatchery conditions in two breeding hapas each stocked with 12 males and 25 females. Eleven microsatellite markers were used to estimate the reproductive success of individual males and females. In total, 760 offspring from 76 spawnings were included in the paternity analysis. Mating patterns in Nile tilapia ranged from single pair to promiscuous mating. Multiple paternity was detected in 70% of the broods, with up to 4 males fertilizing a single clutch. Multiple maternity was also detected in over 30% of the clutches analyzed. There was a very high variance in male reproductive success, with one third of males siring more than 70% of the offspring. Male condition factor had a significant effect on reproductive success with larger males siring a large proportion of offspring. The high reproductive variance resulted in a Ne of around 16 for hapas A and B, and a low Ne/N ratio, 0.43 and 0.45 or hapas A and B, respectively. The rate of inbreeding, F, for each hapa, was estimated to be around 3.0% per generation, which is about twice the inbreeding expected in an idealized population of the same census size. When designing fish breeding programmes based on mass spawning, the higher inbreeding and lower effective population size should be taken into consideration.