Atyid shrimps are often an abundant component in undisturbed tropical streams. Studies in coastal streams in Puerto Rico and Brazil have demonstrated the importance of this group in removing periphyton and sediment from hard substrates and their effects on the composition and quantity of periphytic algae. We used experimental exclosures to investigate the influence of the small atyid Paratya australiensis on periphyton accrual on hard substrates in a coastal stream in the subtropics of Australia. We measured organic and inorganic matter, chlorophyll and algal biovolume in the presence and absence of shrimps on natural and artificial substrates. We found a 5-fold increase in the amount of organic matter on natural substrate in the absence of P. australiensis and a two to 10-fold increase in total periphyton mass on artificial substrate. The natural substrates did not show differences in biovolume of algae, however, algal biovolume on the artificial substrates was significantly higher in the exclusion treatment and diatoms were most affected. We conclude that P. australiensis can be considered a strongly-interacting element of the stream biota and an important species for monitoring and conservation.