SUMMARYIt was suggested that the leaf mark polymorphism of Trfolium repens might be important in visual selection of diet by sheep. Sheep with rumen fistulae were allowed to graze in restricted areas of permanent grassland and the morph frequency in the rumen was compared with that on offer in the pasture. There was clear preference by the sheep for unmarked leaves and some evidence that suggests apostatic selection between the various forms of mark.