On the irregular occurrence of Littorina neritoides along the Dutch coast The small periwinkle (Littorina neritoides) occurred commonly along the Dutch coast in the years 1949-1955, but was not found after 1956, until rediscovered by Stock in 1975. Also along the French North Sea coast, in the German Bight and in Denmark the species is not always present. This unstability can be explained by assuming that the pelagic egg-capsules of L. neritoides, which are present in the plankton in winter, do not tolerate the then prevailing low temperatures, except in very mild winters. Attention is drawn to the correlation which exists between the presence in Dutch waters of L. neritoides and that of some crabs, notably Portumnus latipes.