The bycatch from shrimp trawling has become a matter of concern due to the impact of fisheries on marine ecosystems. Several studies have emphasized the impacts of shrimp trawling on fish and invertebrates, but very little is known about the effects on seaweed communities, despite their critical role in the structure and function of marine ecosystems. In this study we assessed the taxonomic composition and biomass of seaweeds in the catches of 66 shrimp trawls sampled by observers at the fishing zones located north and south of the Magdalena river mouth, Colombian Caribbean, between July and September 2004. A total of 16 taxa of seaweeds were identified. Average seaweed biomass (0-0.15 kg.h-1 north y 0-10 kg.h-1 south) was higher in the south fishing zone and was consistently lower than shrimp and discard biomass in both fishing zones. The presence of macroalgae in shrimp trawl bycatch and the continuous fishing activity during almost five decades in the Colombian Caribbean stress the need to make modifications to the trawl gears to reduce negative (abrasive) effects on benthic communities.