A novel method was developed for the screening and detection of fucoidanase activity in bacterial colonies using fucoidan–agarose plates and staining with hexadecyltrimethylammonium bromide (Cetavlon). Colonies with fucoidanase activity were indicated by a transparent halo. The medium containing undegraded fucoidan showed as a milk-colored background, but areas with degraded fucoidan under the colonies were visualized as a transparent halo.