From fish samplings (rotenone: 6 February 2002 & 13 May 2003; boxtraps: January 2008 to July 2009) in a mangrove lagoon, the demographic structure, reproductive period and diet of Lophogobius cyprinoides were studied. Individual fish were measured (< 1 mm), weighed (± 0.01 g), and sexed by observing the urogenital papilla, sex-ratios and total length (TL)-standard length and length-mass relationships were calculated. Five individuals per size class (2 mm) were used to study sexual maturity by macroscopic observation of gonad structure with a sexual maturity scale, and diet by qualitative and quantitative examination of digestive tract contents and calculation of the following indices: levels of fullness and vacuity; occurrence, number, and point frequencies of various food items, and quality of nutrition. Growth is isometric and sexual dimorphism in favor of males (78 vs. 64 mm), demographic structure (18 ≤ TL ≤ 78 mm) and condition evolved over time, with a distinct femininization of the population. Sexual inversion appeared from 46 mm onward, and total masculinization at 72 mm. Protogynous hermaphrodism is advantageous in a sedentary, subcryptobenthic species that is reproductively active almost throughout the year. The species has an omnivorous diet specialized in copepods and plants