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Reproductive biology of the common ponyfish Leiognathus equulus was described based on 958 specimens caught by otter trawlers in the south-western waters off Taiwan and landed at Tungkang and Linuan fishing ports from March 2000 to February 2001. Macroscopic appearance of the ovary, gonad-somatic index, oocyte diameter frequency distribution, and histological examination suggested that the spawning season of the common ponyfish in south-western Taiwan waters is from May to August. Eight stages of oocyte development were determined based on histological examination, and ovarian development can be divided as immature, early maturing, late maturing, mature and spent stages. The sex ratio of 0.52 was not significantly different from 0.5 usingχ2 test, but females were overwhelmingly more when fork length (FL) was greater than 170 mm. The relations between fecundity (F), batch fecundity (BF), and body weight (BW) were estimated to be F =−5328 + 1.2946 BW ( n = 132, P < 0.05) and BF = −11.26 + 0.2748 BW ( n = 25, P < 0.05), respectively. Mean fecundity and mean batch fecundity (± standard error) were estimated to be 129 955 ± 79 343 and 28 160 ± 14 698, respectively. Sizes at 50% maturity estimated from the Logistic model were 162 mm FL and 158 mm FL for females and males, respectively. [ABSTRACT FROM AUTHOR]