Summary Length–weight and length–length parameters were calculated in the Talkhehrud River, northwestern Iran, for four freshwater fish species belonging to two families. The b values varied between 2.27 and 3.19, with a mean ± SD of 2.2777 ± 0.387. The b parameter values in the length–weight relationship equations were determined as 2.73 for Alburnus atropatenae Berg, 1925; 3.19 for Alburnoides petrubanarescui Bogutskaya & Coad, 2009; 2.92 for Capoeta capoeta (Güldenstädt, 1773); and 2.27 for Oxynoemacheilus persa (Heckel, 1847). This study provides the first reference on the LWRs and LLRs for these species in the Talkhehrud River.