Scomberomorus sierra (Pacific sierra) is one of the most important species in the artisanal fishing on29-4430 Boletín de Investigaciones Marinas y Costeras • Vol. 46 (1) • 2017the Colombian Pacific. In order to investigate its reproductive biology, important input for fisherymanagement, gonadal development was described and partial fecundity and average size of the oocyteswere estimated. Fish samples came from commercial landings on different fishing grounds. Total (TL)and standard lengths of each fish were measured, as well as the total and gonadal weights. Fecunditywas estimated using a gravimetric method combined with image analysis performed using the openaccess software Image J®. 79 fish were collected between November 2013 and April 2014, rangingfrom 39.0 to 85.0 cm in TL and weighing between 300 and 2850 g. The allocation of gonadal maturitywas histologically verified, finding 75 females and 4 males. From the measurement of 12084 oocytes, anaverage diameter of 0.551 ± 0.024 mm was obtained, while fecundity varied between 52398 and 480803oocytes per gonad. Microstructurally, four stages of oocyte development (perinuclear, cortical alveoli,vitellogenic and hydrated) were identified; only one state was observed in males. Mean diameter of theoocytes in the cortical alveoli, vitellogenesis and hydrated stages were 168.75 ± 3.86 μm, 324.08 ± 5.89μm and 388.90 ± 8.81 μm, respectively. The relationship between fecundity and morphometric variablesof the fish was significant and positive, evidencing that the largest females are the most fertile. Size atsexual maturity was preliminary estimated at 58.88 cm TL. The results obtained are inputs from fisherymodels with direct applicability to management of S. sierra.