en Ecology Ecology پژوهشي Orginal research papers <em>Pristinella jenkinae </em>(Stephenson, 1931) is a freshwater cosmopolitan oligochaete. This species was found during limnological investigation in two rivers alongside Iranian coasts and has not been previously reported from Iran&rsquo;s freshwater fauna and Southern Caspian Sea basin. The specimens of <em>P. </em>jenkinae were collected bimonthly from Cheshmehkileh and Sardabrood rivers and estuaries (river mouth) using a Van Veen grab (0.03 m²) and Surber (0.1 m², 0.2 mm-mesh size) samplers at three stations in each estuary (S1: riverine, S2: estuary and S3: marine ecosystem) with three replicates from November 2014 through September 2015. Results of temporal distribution showed that the highest and lowest density and biomass of this species were in January (102.3<span dir="RTL">&plusmn;</span>68.3 ind m^-2 and 0.075<span dir="RTL">&plusmn;</span>0.034g m^-2) and in September (24.4<span dir="RTL">&plusmn;</span>12.3 ind m^-2 and 0.020<span dir="RTL">&plusmn;</span>0.005g m^-2), respectively which were significantly different (N=57, t=0.99, <em>p</em><0.05). Spatial distribution of <em>P. jenkinae</em> among sampling stations (S1, S2 and S3) showed significant differences (N=57, t=0.99,<em> p</em><0.05). Freshwater stations (S1) within the river showed higher density and biomass (112.1<span dir="RTL">&plusmn;</span>64.8 ind m^-2 and 0.082<span dir="RTL">&plusmn;</span>0.035g m^-2) than semi-brackish stations (S2) within the estuary (18.8<span dir="RTL">&plusmn;</span>10.3 ind m^-2 and 0.013<span dir="RTL">&plusmn;</span>0.005g m^-2). Semi-brackish stations (S2) showed higher density and biomass than brackish stations (S3) within the sea (0<span dir="RTL">&plusmn;</span>0 ind m^-2 and 0<span dir="RTL">&plusmn;</span>0g m^-2). Density and biomass of this species in Cheshmehkileh River and estuary was more than Sardabrood. A significant correlation (N=57, r=Pearson, <em>p</em><0.05) between density and biomass of <em>P. jenkinae </em>with environmental variables was found. Pristinella jenkinae, Distribution, Riverine, Estuary, Caspian Sea 109 128 http://jifro.ir/browse.php?a_code=A-10-2237-3&slc_lang=en&sid=1 M. Erfani meysamtavoli@yahoo.com 100319475328460039189 100319475328460039189 Yes Coldwater Fishes research Center, Iranian Fisheries Science Research Institute, Agricultural Research Education and Extension Organization