Reproductive biology of gray-eel catfish, Plotosus canius was studied at Pattani Bay by determining its year round changes of gonadosomatic index (GSI). Results show that the GSI was maximum in October (8.4 %), and minimum in April (0.24 %). The sampled size of sexually matured female fishes ranged from 40.9 to 60.5 cm in total length, and between 367.56 to 1,159.37 g in body weight. The average total length of the female fish is 51.9 cm, with an average body weight of 788.8 g. The average fecundity was 1,842.14 egg/fish. The average GSI was 10.87 % (n=14).The fish total length (TL) and body weight (BW) of the matured fish were linearly related, and could be represented by the linear regression as: BW = 32.295 TL – 887.52. The relationship between body weight (BW) and fecundity (Fe) could also be represented by the linear regression as Fe = 398.67+1.83BW. The eggs were adhesive demersal egg type, and rounded. The distribution of egg’s diameter could be categorized into five groups i.e. group1 (3.3%), group2 (6.9%), group3 (11.0%), group4 (54.1 %), and group5 (24.6 %), with values of 2.89± 0.19 mm, 3.44±0.36 mm, 4.65±0.28 mm, 5.65±0.26 mm, and 6.38±0.25 mm respectively. The spawning ground is made of silty clay soil, with sediment’s pH of 7.16±0.47and water salinity of 12.06 ± 0.78 ppt. We also found that the sperms activity was most active in the 0.9 % sodium chloride’s concentration level.
