Abstract: This study aim to find the colonizing ability of cellulose degrading fungi which were derived from terrestrial sources, in sea water with salinity of 34‰, which can be used in degradation of marine algal debris in order to use as fertilizer. Algal debris is rich in minerals but due to its salt content it takes relatively longer time to degrade by microbes. Here the fungus Cladosporium sp. depicted the highest activity of 64.01 FPU/ml. which was 20.74 FPU/ml in CBM medium on the same substrate; this shows their preference as well as tolerance to marine environment.