Certain species generally regarded as calcifuge in the British Isles are of widespread but local occurrence on shallow soils over calcareous substrata. The shallow soils invaded by calcifuges are not in all cases surface-leached. Calcifuge species have not been observed on limestone or chalk soils which are subject to severe drought. They do not, however, invade certain calcicolous communities on continuously moist soils. No evidence of ecotypes adapted to survive on calcareous soils is yet available for calcifuge species. Experimental evidence suggests that in some of the situations where calcifuges survive on unleached limestone soils, certain nutrient deficiencies, to which they are susceptible on calcareous soils, are offset by the accession of constitutents from other soils and by circumstances leading to reduced aeration. Some calcifuges may persist in calcicolous communities in spite of nutrient deficiencies comparable with those developed in cultivation on calcareous soils which the same species fail to colonize naturally. Additional factors such as "interference" or drought do not appear to be operative in these situations.