Mesozooplankton biomass in the upper 1000m of the Andaman Sea was studied as part of the Marine Research-Living Resources (MR-LR) assessment programme during February 1999 providing baseline information on stratification of mesozooplankton of the area. The distribution of mesozooplankton biomass is related to the oxygen minimum layer (<0.1 ml/litre). The results show that biomass and group wise diversity were maximum in the mixed layer (average 5 ml/100m3) and sometimes in the thermocline layer (average 4.4 ml/1003). A prominent feature was that the minimum average biomass values were confined to the depth strata of 300m-Bottom of thermocline (BT) (0.9ml/100m3) where the oxygen minimum layer was more pronounced. The averages of zooplankton group percentages in the upper 1000m showed that the copepoda forms the major group (81.01 %) followed by the chaetognatha (6.7 %), ostracoda (4.8 %), siphonophora (1.9 %), and decapod larvae (1.3 %) and the corresponding average population densities were 4,620 /100m3, 1,201/100 m3, 104/100 m3, 102 /100m3, and 7 /100m3