Benthosema pterotum is a dominant myctophid in the shelf region of the East China Sea (ECS). They are a key species linking secondary producers and upper trophic levels. We examined the reproductive biology of B. pterotum in the ECS, based on 3065 specimens (10.7–54.8 mm standard length (SL)) sampled during cruises in 1999 and 2004–2009 to enable data over a 12 month cycle to be assessed. Between 14 and 30 mm SL, the ratio of females to males was ~1:1; it increased with SL, and all individuals ≥46 mm SL were females. The gonadosomatic index (GSI) at 50% sexual maturity of females and males was 3.91 and 0.24, respectively. Based on this criterion, females and males can mature at sizes larger than 28 mm and 24 mm SL, respectively. Although mature females having GSI ≥3.91 occurred over nine months (May–January), mean GSI of females peaked sharply during August–September, i.e. the primary spawning period. The hepatosomatic index peaked during April–July, and declined during August–September, indicating storage of materials and energy for reproduction. Based on data from seasonal bottom trawl surveys, small individuals of B. pterotum began to occur abundantly in autumn, and modal body lengths increased progressively during spring–summer, corresponding to the above reproductive seasonality. Oocytes at various sizes were found in mature ovaries, indicating that they are multiple spawners. Egg size at hydration ranged from 0.5 to 0.85 mm. Batch fecundity was positively correlated with SL, and ranged from 253 to 1942 eggs in fish from 30.1–54.8 mm SL.