Marine biofouling occurs on the bottom of boats, fishing gear, nets and the surface of underwater construction materials, as well as on the surface of living and nonliving marine organisms. It causes great damage to marine and fisheries industries. This paper discusses some of the biology of marine biofouling organisms and the relationship between marine biofouling and fisheries. Special attention is given to the damage caused by marine biofouling to the fin-fish cage culture industries of Hong Kong. At present, approximately 5,000 species of marine biofouling organisms occur in the world, of which 2,000 species are recorded. In the coastal water of China, there are about 700 recorded species. About 250 biofouling organisms are found in Hong Kong. The dominant species are barnacles, bryozoans, polychaetes, sea squirts (ascidians) and bivalves. The main biofouling of fish cages in Hong Kong is caused by the polychaete Hydroides elegans, sea squirts Styela plicata and S.canopus, oyster Ostrea glomerata, barnacles Balanus reticulatus and B.trigonus. Depending on habitat and sea water temperatures, marine biofouling has obvious seasonal fluctuations in growth and occurrence. Marine fouling organisms occur on the fish cages in Hong Kong all the year round, but April to June is the predominant season of the occurrence of Hydroides elegans. During this period, small mesh nets will become completely obstructed by this animal. Oyster Ostrea spp., barnacles Balanus reticulatus and B.trigonus and the sea squirts Styela plicata and S.canopus occur in the months of June to November. From November to March the main fouling species is the bryozoan Bugula neritina. In Hong Kong because of marine fouling, fish cage nets need to be changed about every 1-3 months. The biofouling organisms (such as H.elegans) which attack nets, can be sprayed with sea water which will clean the net. But those with calcareous substances such as Ostrea and Balanus must be cleaned by sunlight and manpower. At present, antifouling paints are being tested on fish cage nets in China, Japan and Hong Kong. However, toxic substances such as TBTO and TBTF from these paints may accumulate in the fish with direct or indirect effects on human beings. So their use requires further investigation.