Data from algae living on rock surfaces were analysed by ordination techniques. Quantitative data were less useful than qualitative data. Splitting the data, by association-analysis and by discontinuities on the ordination diagram, gave more precise ordinations of fewer stands. A standardization to correct the effect of species number was successfully applied. Environmental factors varying from microscopic range to 100 yd (91 m) were shown to be correlated with the occurrences of algae.