Abstract
During the past few months we have collected a considerable amount of information relating to the value of pH. as affecting the distribution of the three species of Limnaea commonly found in North Wales. Tables I and II show that L. truncatula occurs over a range of pH. 6·0 to 8·6, and L. peregra 5·8 to 8·8; L. palustris was recorded from 7·4 to 8·0 (but only three of the readings included this species).