The Wall Brown ( Lasiommata megera) is becoming increasingly rare in the Netherlands. The situation is better in the western part of the country. The ecology of the caterpillar of the Wall Brown has been studied in this part of the country. The caterpillars were found on different grass species. The main host plants were Festuca rubra, Dactylis glomerate and Arrhenatherum elatius. The host plants grew on sheltered places and more or less facing south, in front of vertical structures, like walls and trees. The caterpillars can be active at very low temperatures due to the warmer microclimate of these places.