Mysids from a tropical estuary (Cochin backwater) in India were studied based on samples collected over a period of one year. Four species belonging to three genera) were represented: Mesopodopsis orientalis, Mesopodopsis zeylanica, Rhopalophthalmus indicus and Kochimysis pillaii, among which Mesopodopsis zeylanica was the most dominant species, recorded throughout the year with its peak abundance observed during the monsoon period (June to September). The wide range of fluctuation in their population density was considered to be mainly due to their heterogeneous distribution. Reproduction of M. zeylanica in Cochin backwater is continuous throughout the year. The number of embryos carried by a single female ranged from 7 to 12, and was correlated with female length (p<0.05), tending to increase with increasing the size of the female. Egg size varied between 0.38 and 0.43 mm, with no correlation with length of the female, and size differences were observed in the same brood. Males and females attain sexual maturity after reaching a total length of 5 mm and 4.5 mm respectively. The environmental parameters, salinity and chlorophyll a, have much influence on the population density of mysids.