Field investigations were carried out on the growth and reproduction of two species of the brown algae Sargassum aquifolium (Turn) J. Agardh and S. asperifolium (Her et Mart.) J. Agardh growing at Pange Island, Zanzibar between May 1983 and December 1984. A general decline in size and leafyness of the plants was noted between July and December. S. asperifolium disappeared between September and October 1983 and July-November 1984. Many S. aquifolium stipes on the other hand persisted, but suffered much attrition and had minimum size between November and December. Decline in stipe size in both species was attributed to be mainly due to disintegration, leaf and receptacle shedding of the thalli of old mature plants. Many thalli were reduced to mere leafless stumps between August and early December. From late December, positive size increase was observed. This resulted from the fast growing buds of pre-existing stumps as well as fresh germlings. S. asperifolium which had virtually disappeared between September and October 1983, July - November 1984, started appearing at the end of November, and by January had acquired a higher average size in comparison to S. aquifolium. In general the two species appeared to grow best between January and June. They then degenerated between July and early November. Reproduction was most pronounced during the best growing season.