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This study focuses on the population biology of the common guitarfish Rhinobatos rhinobatos, a cartilaginous fish listed as Endangered in the International Union for the Conservation of Nature (IUCN) Red List. Between December 2012 and January 2014, 67 individuals were collected by bottom longlining in coastal Lebanese marine waters at different ports at depths ranging from 10 to 110 m. The total length (L(T)) of the specimens ranged from 50 to 143 cm, and the mean ± s.d. was 76.2 ± 19.7 cm. The most common L(T) classes were between 60 and 70 cm. The total mass of the specimens ranged from 410 to 10,000 g, and the mean ± s.d. was 1841 ± 1987 g. A total of 34 males and 33 females were collected, and the sex ratio was not significantly different from 1:1. The mass and L(T) relationship showed positive allometric growth (b = 3.096 and r(2)  = 0.99), and the mean ± s.d. L(T) at which 50% of the individuals were sexually mature was 84.73 ± 5.81 cm for females and 78.57 ± 4.88 cm for males. The gonado-somatic and hepato-somatic indices were determined along with a condition factor, and parturition appeared to occur in winter. The primary prey items found in the fish stomachs during the autumn and winter seasons were Penaeidae. The results of this study will help to parameterize models of the population dynamics for this exploited fish stock to ensure the long-term sustainability of its fishery., (© 2016 The Fisheries Society of the British Isles.)