Length–weight relationship and condition factor of the Characidae matrinxã, <italic>Brycon falcatus</italic> (Müller & Troschel, 1844), in the Teles Pires River, southern Amazon.

Summary: The aim of this study was to verify whether the in situ supply of soybean as a fish attractant bears any influence on the length–weight relationship and condition factor of the omnivorous fish species, <italic>Brycon falcatus</italic>. In addition, estimated was the length–weight relationship, standard and total length, condition factor, and size of <italic>B. falcatus</italic> in the spawning period in the Teles Pires River Basin. In the experimental design, rivers of the Teles Pires Basin containing soybean attractant (Verde River, Celeste River, Teles Pires River and the Tapaiuna River) were mapped. Fish were also collected from the Cristalino River, which contained no soybean attractant (treatment control). Samples were collected from August 2012 to July 2013. Length–weight relationship of <italic>B. falcatus</italic> in the Teles Pires River Basin can be obtained by the expression: Wt = 0.0091*SL3.376, <italic>r</italic>2 = .97, <italic>n </italic>=<italic> </italic>102, and length‐length relationship: TL = 1.128*SL + 3.978, <italic>r</italic>2 = .96. The condition factor of all specimens collected in the Teles Pires Basin was <italic>k</italic> = 1.055. Size at capable phase (L50) of spawning was estimated at 23 cm standard length. Results showed no significant difference in the length–weight relationship or condition factor of <italic>B. falcatus</italic> collected from rivers with or without soybean attractant. Concluded is that the in situ supply of soybean as a fish attractant does not interfere with the wellbeing of <italic>B. Falcatus</italic> in the Teles Pires River Basin. [ABSTRACT FROM AUTHOR]