<b>Background and Objective:</b> Studies on fish biology are essential for the sustainable management and conservation of fish biodiversity. This study observed the length-weight relationships and condition factors of common rudd fish, which give us some information about the ecological and nutritional conditions of fish. This information is crucial to fishery management. <b>Materials and Methods:</b> The length-weight relationship (LWR) and condition factors (K) of 270 <i>Scardinius erythrophthalmus</i> caught in Anzali Lagoon from April to August, 2015. <b>Results:</b> The total length and body weight of <i>S. erythrophthalmus</i> ranged from 94-179 mm and 11.98-98.5 g, respectively. A strong positive relationship was observed between length and weight (r = 0.96). There were no significant differences in the condition factors of males and females (p>0.05). The lowest K value was observed in April (K = 1.5±0.04) and the highest in July (K = 2.25±0.34). The sex ratio of 1.0 male: 1.15 female was observed, with no significant difference between males and females (p>0.05). <b>Conclusion:</b> In this study, the regression coefficient (b) in males and females was 3.35 and 3.29, respectively, more than 3.0, which means they have positive allometry growth in both sexes.
Keywords: Anzali Lagoon; Length-weight relationship; Scardinius erythrophthalmus; condition factor; fish biodiversity.