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Background. Loach fishes, Cobitis sp., are found in the majority of streams in the southern Caspian Sea basin, but reproductive parameters of them are barely known. Reproductive conditions are key parameters of fish populations and their assessment is very important for several reasons. Therefore, the main aim of this study was to obtain reproductive characteristics of this fish which will be a first report for the basin. Materials and methods. Sampling was done at monthly intervals throughout the year and 226 individuals of Cobitis sp., were caught from the Babolrud River in the southern Caspian Sea basin, north of Iran. Age, sex ratio, fecundity, oocytes diameter, gonadosomatic- and modified gonadosomatic indices were estimated. Regression analyses were used to find relations between fecundity and fish size (length and weight), gonad weight, and age. Results. Sex ratio differed significantly from unity, and the percentage of females was greater than that of males. The mature females and males were longer than 45 and 35 mm in total length (+2 and +1 in age, respectively). The average egg diameter was 0.58 mm. The spawning took place from beginning of May to late July, when water temperature was between 19.1 and 24.6°C. Average GSI value at the beginning of the reproduction period was 8.93% and ranged between 4% and 26% in ripe, mature females. The average of absolute and relative fecundity was calculated 2172 and 590, respectively. The absolute fecundity was significantly related to body weight and gonad weight. Conclusion. Based on the pattern of gonadosomatic index, it was concluded that this fish has a prolonged active reproductive period. Their reproduction in this basin shows some differences from other reports, which might be related to different environmental conditions. [ABSTRACT FROM AUTHOR]