This study describes the length-weight (LWR) and length-length (LLR) relationships for 254 specimens of Gambusia holbrooki from the Anzali Lagoon, in the southern Caspian Sea basin. Total length (TL), standard length (SL) and weight were determined. Specimens were collected occasionally in summer 2020 by hand net. The maximum total length and weight of the studied population were 5.6 cm and 3.5 g, respectively. The length-weight parameter b for this species was 3.02, with a high regression coefficient (r2>0.95). The LLR was highly significant (r2>0.99). [ABSTRACT FROM AUTHOR]