INTRODUCTION According to Holthuis (1950a) the family Astacidae is represented in the Netherlands only by one species, Astacus astacus (L., 1758). Apart from a detailed description of the external anatomy, he gave the following data. The average size of the adults amounts to 80 mm, however, the maximum size sometimes reaches up to 250 mm; the colour is brown-green or olivegreen. Astacus lives in running as well as in stagnant water with good shelter. The food consists of Mollusca, Crustacea, worms, insect larvae, tadpoles and sometimes of vegetable matter or dead organisms. Fertilization takes place between medio October and the end of November; the female carries her eggs from November up to June or July. Holthuis (1950a) recorded 38 localities, situated all in the southern and eastern part of the country, where Astacus has been found during the period from 1660 to 1947. In his opinion Astacus astacus was formerly rather to very common, but as a result of the pollution of brooks and perhaps also of the crayfish-pest, it has disappeared in many places. However, Holthuis (1950b) got the impression that the population of Astacus was improving again in the Netherlands as well as in Belgium and France. Mohren (1970) reported that he had observed in the Julianakanaal (province of Limburg, Netherlands; see fig. 2) thousands of crayfishes when the water level was lowered for repairs of sheet-pilings. At Urmond on the Julianakanaal these animals were sucked in by a pump for industrial cooling water of "De Staats Mijnen" (D.S.M. = The Netherlands National Mining Company). At the end of 1972 Mr. F. van Herp and I discovered that these crayfishes