Seasonal variations in the taxonomic composition, number and biomass of the basic mesoplankton groups of the Azerbaijani sector of the Caspian Sea have been investigated. It was shown that the species composition and quantity parameters are dependent from water temperature, hydrologic conditions and biological rhythms of hydrobionts. The average number of mesoplankton was 3604 specimen/m3 and biomass – 67.21 mq/m3 . The largest contribution to the biomass provided by Acartia clausi. The investigation carried out in 2006-2010 years showed that the quantity parameters of mesoplankton increased from year to year.