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The spatial distributions of porosity and fractional dimension of autotrophic and heterotrophic/autotrophic biofilms applied by direct current in the range of 0-25 A/m2 were investigated by using the micro-slicing technique and image analysis. The results showed that the porosities of the autotrophic and heterotrophic/autotrophic biofilms respectively decreased from ca. 92% and 96% in the top layer to ca. 54% in the bottom layer while the fractional dimensions increased from 1.1 and 1.05 in the top layer to 1.4 in the bottom layer in the absence of current. The increase in the current density decreased the porosities and increased the fractional dimensions in the top layers. In the deeper layers, the current had insignificant influence on the porosity and fractional dimension.