The present work was carried out to compare some biological aspects, chemical composition, fatty acids, minerals and heavy metals of the commercial Rabbit fishes Siganus canaliculatus inhabit the Arabian Gulf and Siganus rivulatus inhabit the Red Sea in Saudi Arabia. Fish samples were selected to represent all fish size categories in the catch and were collected during the period from November 2002 to March 2003. Each fish specimen was measured, weighted and recorded, then dissected to investigate the fullness of alimentary canal and the maturity stage of the gonads. The length-weight relationships were computed by simple regression analysis and graphically represented. Both of absolute and relative condition factors (Kc and Kn) were estimated and graphically represented. The muscles (flesh) were removed and weighted to estimate the flesh percentage. Samples of flesh were taken to determine moisture, protein, ether extract, ash, fatty acids, minerals and heavy metals. Some samples of the entire fish were used to proximate analysis. The present results indicated that the Kc, Kn, maximum total length and flesh percentage of Rabbit fish Siganus canaliculatus were'higher than that of Siganus rivulatus. In general, polyunsaturated fatty acids were relatively low and heavy metal concentrations in the flesh of the two Siganus species-were lower than the permit limits for Human consumption,