A chromatographic study of fiuorescent substances in the body-surfuce mucus of lyinnaeid snails has shown the existence of a number of ctirointttogruphically distinct forms of Lymnaea peregra. Tho Auorescent patterns of these forms are doscribed arid illustrated and their geographical distribution is mentioned. Evidence of the genetically controlled nature of these patterns is given and tho occurrence of natural hybrids is recorded. The practical uses of the technique in determining the genetic hnogonoily of populations are discussed, also the possible significance of the geographical distribution of the forms.