Prochilodus magdalenae has historically been one of the most important food sources for the human population nationwide. This Colombian endemic species represents more than 50% of all catches in the main river basins of the country. The isolation of the population of P. magdalenae in the Atrato basin demands the adoption of conservation measures adapted to their biological and reproductive characteristics. During our study we recorded the biometric information of P. magdalenae catches obtained via artisanal fishing in the Marriaga swamp (Atrato River lower basin) from April 2014 to March 2015. From a total of 574 recorded individuals, 427 were females and 147 males. The female size ranged between 200 and 415 mm (standard length, SL), and the male size ranged between 210 and 415 mm (SL). In females, the length-weight analysis indicated an isometric growth (b=2.95) and a high correlation between the two variables (r2=0.81). In males, it indicated a negative allometric growth (b=1.87) and a low correlation (r2=0,56). The average size at maturity for the whole analyzed population was 365 mm (SL). The highest period of reproductive activity was estimated between October and March. The size ranges, the average size at maturity, and the spawning seasons of the individuals registered in the Marriaga swamp differed from those registered in the middle and lower area of the Atrato river basin, and other basins where the P. magdalenae has been studied. © 2019. Acad. Colomb. Cienc. Ex. Fis. Nat.