1. The haploid chromosome number of the freshwater snail Planorbarius corneus from the Netherlands is 18, the diploid 36. The haploid number agrees with that found for this species in France by LE CALVEZ & CERTAIN (1950), but differs from the number (n = 17) reported by AZEVEDO & GONÇALVES (1956) for this species from Portugal. 2. Chromosome numbers have a very limited value for discriminating species in the euthyneuran gastropods, but can be used in regard to the higher taxonomic categories as an indicator of degree of specialization.