The knowledge about algal chemistry is gradually becoming more significant. Recently a new term, phycochemistry has been proposed by Shameel (1990). Phycochemistry is the study of natural products and chemical constituents occurring in algae, from a biological point of view. As a new branch of phycology it has emerged as a result of huge and amassed knowledge about secondary metabolites occurring in algal phyla (divisions). It distinguishes itself very clearly from phytochemistry in general on one hand and algal biochemistry on the other. It differs from the main objectives of algal chemotaxonomy and should therefore, be dealt with as an independent branch of phycology. The present study was undertaken to investigate the predominant species of blue-green, green, brown and red seaweeds and to bring about a comparison of the composition of their saturated and unsaturated fatty acids and sterols as well as their bioactivities. The idea was to develop a better understanding about their phycochemistry.