The aim of the current study was to address length‐weight relationships (LWRs) in seven ornamental fish species from the Unini River basin—the main tributary of lower Negro River (Brazilian Amazon). Specimens were seasonally sampled by using hand nets and beach seines (mesh size < 1 mm) in five different occasions (August 2009—December 2010). Preserved specimens (initially kept in 10% formalin and transferred to 70% ethanol 10 days later) were measured and weighted in laboratory. The LWR was calculated based on a log‐transformed linear regression. The maximum standard length of each species was different from the ones previously reported at Fishbase. In addition, b values were different from the ones reported for the four species whose data were available in the literature.