AbstractWeekly interflow and permeability data were obtained for 15 sites in two field plots in North Yorkshire. Both plots are similar in terms of lithology, slope, micro‐relief, climate and soil type, one being dominated by rough pasture grasses and the second by bracken. One factor which explained 75 per cent of the variation in interflow was the difference between topsoil and subsoil permeabilities, the former always being the greater. This permeability difference is discussed in terms of soil texture, porosity and plant‐rooting systems. Textural contrasts are considered insufficient to cause the large‐scale variations in permeability observed throughout the year. Being a fixed variable, texture cannot influence a characteristic which varies from one week to the next. Contrasts in total porosity between topsoil and subsoil also effected no discernible influence on measured permeability differences, although measures of association increased as smaller pores were excluded from the analysis. Utilizing only those pores drained at 20 cm of water tension, highly significant correlations were established. In the plot dominated by bracken, greater ‘lateral permeabilities’ are produced by well‐developed systems ofPteridiumrhizomes. As the advancing front of such rhizomes extends forwards at quoted rates of up to 45 cm/yr, the rear portions decay, leaving horizontal ‘tubes’ in the soil. Recognized omissions from this analysis are the effects of soil fauna and desic