Biol. fouling by settlement and growth of organisms are among the most important problems affecting a structure immersed in water.Most anti-fouling approaches rely on biocide-based coatings which release toxic agents to kill microorganisms but pollute the aquatic environment.Betaine-based polymers were synthesized to develop underwater surface modifications and two strategies were designed to resist biofouling.The first is to resist the attachment of microorganism by a non-fouling and non-leaching sulfobetaine-based polymer.The second is a killing-releasing surface modification using a degradable polycarboxybetaine ester that turns from cidal to non-fouling upon hydrolysis.In prior work, settlement, growth and attachment of two marine organisms (green algae and barnacles) showed substantial reduction on sulfobetaine modifications.In this study, betaine-based polymers were developed as underwater coatings and investigated for resistance to biofouling.Their resistance to microbial attachment and biofilm formation in a gram neg. and gram pos. biofilm models achieves 98.3% and 99.3% reductions relative to unmodified substrate, resp.After 112 days of washing with saline, this non-leaching surface modification demonstrated a 99.4% reduction in biofilm, representing no loss of activity over this time frame.The leachables and toxicity of the coatings were analyzed by HPLC, NMR, and biol. assays.Modified substrates were also characterized for chem., phys., and mech. properties to ensure proper alignment with underwater coating requirements.Some of the betaine-based polymers demonstrated a broad spectrum of anti-biofouling performance, long-term efficacy, and non-toxicity to the environment.