Chlorinated fatty acids have been found to be major contributors to organohalogen compounds in fish, bivalves, jellyfish, and lobster, and they have been indicated to contribute considerably to organohalogens in marine mammals. Brominated fatty acids have been found in marine sponges. Also, chlorinated lipids have been found in meat exposed to hypochlorite disinfected water, and in chlorine-treated flour and in products made from suck flour. Following exposure to chlorine bleached pulp mill effluents, aquatic organisms may have elevated concentrations of chlorinated fatty acids in their lipids. However, a natural production of halogenated fatty acids is also possible. In this paper we summarize the present knowledge of the occurrence of halogenated fatty acids in lipids and suggested ways of their formation. In Part II (Trends Anal. Chem. 16 (1997) 274) we deal with methods of their determination.