1. The caterpillars of Philudoria potatoria. L. belong in Holland to two phaenotypes, a light one with a pale bluish-grey ground colour, strongly mottled with pale markings (figured in SEPP 1), and a dark one, which is nearly blackish blue and much less provided with pale markings (figured in practically all text books). 2. The pale caterpillars are confined to reedbeds in the lower western part of the country, the dark caterpillars principally to coarse grasses (e. g. Dactylis) in the higher parts (east and south). Mixed colonies are also known. See map. 3. It is highly probable that the colour forms of the caterpillars are due to environmental factors. From 37 light caterpillars collected in one colony in 1948 about half of them turned to dark ones in the breeding cage (unnatural conditions!), when they cast their skin. (Those which remained pale were possibly already in their last skin when we collected them). 4. The food itself is not the principal cause for the differences in colour of the caterpillars. I suppose that they result from differences in light. 5. The pale caterpillar is not confined to Holland. I saw French specimens in the collection of the Musée d’Histoire Naturelle in Paris, among which was one from Beauvais near Paris. 6. The moths resulting from these two types of caterpillars show clear differences, which can only be attributed to differences in the genetical composition of the colonies. 7. Table I shows that the ♂♂ resulting from pale caterpillars are characterized by a high percentage of yellow specimens and a nearly complete failing of unicolorous red-brown ones, whereas the ♂♂ from the dark caterpillars have not yet produced the yellow form, but not rarely the unicolorous dark one appears among them. 8. Table II shows that there are rather strong differences among the individuals of different populations of pale caterpillars. In some of them, not tabulated, yellow ♂♂ are very rare or even unknown. 9. Yellow ♂♂ are not confined to pale caterpillars. In the collection of Mr. H. B. WILLIAMS at Bramley (Surrey) I saw magnificent specimens resulting from dark caterpillars of a colony south of London. 10. With the colour forms of the females the differences are not so strong, though the pale caterpillars produce more pale females and the brown form inversa Caradja is not attained by them. Only the dark caterpillars produce inversa in a rather small percentage (table III). 11. If the females are divided, however, according to the degree of the markings, clear differences are shown. See table IV. 12. It is evident that Holland is inhabited by a “Pragmites-race” and a “Dactylis-race”, which show clear biological, phaenotypical and genetical differences.