Seasonal and inter-annual variations in the density of the invasive polychaete Streblospio gynobranchiata were noted in the south Caspian Sea when sediment conditions were examined at five and 14 meters depths in 2005 and 2010. There was no clear trend in changing density in line one but in line two, in all seasons except summer, density of S. gynobranchiata decreased significantly. Maximum densities of 2,040 ind./m−2 and 1,013 ind./m−2 were obtained in 2005 and 2010, respectively. Although the percentage of total organic matter as a food resource increased from 2005 to 2010, the density of S. gynobranchiata decreased slightly in the same period. It seems that in this invasion phase the population of S. gynobranchiata has decreased to a balance condition. There is evidence that the changing density of this species is also being affected by other biotic/abiotic factors like intra/interspecific competitors and pollutants.