en Ecology Ecology پژوهشي Orginal research papers <span style="font-size:12pt"><span style="line-height:150%"><span new="" roman="" style="font-family:" times=""><span style="color:black">In this paper, both of growth and size structure of a population of <i>C.</i> <i>fossarum</i> are studied for the first time</span>. The average sizes of females were significantly higher than those in males (P<0.05). Proportions of morphological characteristics to carapace length in male were significantly higher than those in females (P<0.05). No difference was detected between the ratio of RL/CL of both sexes (P>0.05). The size frequency distribution was normal for either sex, with a peak of occurrence in the size classes 5, 6 and 7 (4.0-5.5 mm). In females the growth rate of TL in relation to CL was greater than males and constant (isometric growth) in spring and summer, while in males a decreasing rate (negative allometric) in each two seasons of year was observed. Therefore, as expected, the growth rate of W in relation to CL for female sex was greater in spring and summer. Condition factor was higher in female shrimps (1.56) than males (1.52) (P > 0.05). On monthly based analyses, variations in the condition factor were similar for both sexes, and minimum value was observed in winter. <span style="color:black">Asymptotic length (L&infin;), growth rate (k) and maximum life span estimated of males were 6.1 mm, 0.61 per year and 3.43 years; those for females were 6.62 mm, 0.71 per year and 3.17 years respectively. </span></span></span></span><br> &nbsp; Growth, Decapoda, freshwater shrimp, condition factor index 349 358 http://jifro.ir/browse.php?a_code=A-10-3640-3&slc_lang=en&sid=1 M. Naderi mojtabanaderi1364@yahoo.com 100319475328460051335 100319475328460051335 Yes Payam noor university P. Zare mojtabanaderi64@GMAIL.com 100319475328460051336 100319475328460051336 No Gorgan university