This paper deals with arsenic and lead biosorption by different waste biomasses coming from the marine environment. Shoreline seaweeds and seagrasses were used to adsorb metals from aqueous solutions, under different pH. Experimental tests were performed in order to study the equilibrium of biosorption with suspended biomass. The obtained results confirmed the possibility of using marine macrophyte biomass for heavy metal biosorption and evidenced a strong dependence of lead and arsenic uptake on the macrophyte structure. Brown algae were found to be the best sorbents for lead with a maximum observed lead uptake of 140 mg/g; green algae showed a maximum lead uptake in the range 50-70 mg/g; red algae were the worst lead sorbent, in the investigated experimental conditions, with a maximum lead uptake in the range 10-40 mg/g. As concerns arsenic, the macrophytes had in general good sorption abilities when compared with those of activated carbon. Furthermore red algae, that for lead were not effective, resulted to be the best sorbents for arsenic. This was explained by a different speciation in aqueous solution of lead (II), that is cationic with respect to arsenic(V), that is anionic.