The distribution and some biological characteristics of commercially important giant sea catfish, Arius thalassinus (Rüppell, 1837) were studied at Mafia Island. Artisanal fishing catches were sampled, caught mainly with longlines, shark nets and ring nets. These yielded a total of 2 723 kg of A. thalassinus, comprising 756 individuals, the largest measuring 1 000 mm TL. Arius thalassinus occurred only on the western coast of Mafia Island and the highest catch rate was 19.3 kg.fisher-1.day-1 in March when murky water was predominant. The reproductive biology of A. thalassinus was investigated to assess its vulnerability to fishing. Arius thalassinus reached a size at first maturity (LM50) of 520 mm TL and exhibited a low mean (±SE) fecundity of 65.6 ± 3.37 eggs per female within the size range of 605-970 mm TL. The hydrated oocytes were large (mean diameter ±SE = 15.2±0.12 mm). Arius thalassinus spawned once in the study year during February and April, during heavy precipitation (124-499 mm). We therefore conclude that its restricted distribution, large size, low fecundity, late maturation and its reported high trophic level indicate that it would be vulnerable to fishing pressure. It is therefore recommended that fishing for A. thalassinus be restricted during its spawning season to ensure its sustainability