Abstract: The present study was conducted on aspects of reproductive biology of mudskipper, Periophthalmus papilio (Bloch and Schneider 1801) in the mangrove swamps of Lagos lagoon in Nigeria. P. papilio is an economically valued fish species both as food for the populace and baits for industrial or offshore fisheries. About 2167 individuals were caught from the lagoon with non return valve traps between May 2006 and May 2008. Data on sex, Total Length (TL), gonad (gW) and body Weight (bW) measurements were carried out on individual specimen. These data were used in the determination of sex ratio, Gonadosomatic Index (GSI) and maturation in this species. The sex ratio of 1 male to 1.42 females was observed but not statistically different but a departure from the theoretical 1 male:1 female sex ratio. The females were significantly more than the males. The eggs were as small as 0.20 mm and as big as 0.50 mm (mean = 0.36±0.01 mm for average fish of 116 TL). The GSI values varied from 0.01-0.48% in males and between 0.11 and 8.40% in females. Seven stages of maturity: immature, immature and developing, ripening, ripe, ripe running, spent and resting and recovering were encountered. Male fish matured at 70 mm TL and females at 60 mm TL. However, both sexes were at the ripe stage at 90 mm TL. In conclusion, females were significantly more than the males. Between 0.01 (in males) and 8.40% in (females) of the body mass was useful in gonad development. The maturity of females was earlier than the males. The maturity stages obtained from the present study were in conformity with those reported in other teleosts. Therefore, the sex ratios, egg diameters, gonadosomatic index and maturation of the species in Lagos lagoon were investigated to provide baseline data for future ecological and biological studies of this species and other species within the estauaries and coastal waters.