The marine siliceous boring sponge Cliona boring into carbonate substrates of biogenic or non-biogenic origin produces characteristic particles of 15 to 100 micron in size. These particles were quantitatively determined without difficulties by grain size analysis using a scanning electron microscope (SEM). In the Persian Gulf (Iranian coast) and in the Northern Adriatic Sea (Istrian coast) the portion of the fine grained material produced by Cliona is 2 to 3% of total sediment. In the lagoon of Fanning Island/Line Islands (Pacific) this portion amounts to 30% of the total sedime