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1. It was found that larvae of the genera Trichostrongylus, Haemonchus, Ostertagia and Chabertia migrated upwards on the blades of sedge (Kylinga sp.) and of rye grass (Lolium perenne) in greatest numbers when subjected to a light of an intensity about 62 f.c.2. Moisture on the grass was found to favour laval migration. More than 0·12 ml. of water per sq. cm. of soil surface, however, retarded movement. More than 0·2 ml. per sq. cm. tended to prevent larval migration up the grass.3. Of the genera examined Ostertagia spp. were most favoured by the presence of moisture on the grass; Haemonchus contortus was most hindered by its presence.4. Increasing quantities of moisture in the soil up to 85% saturation assisted larvae to move on to the grass. Above this value larval migration was retarded.5. Ostertagia spp. were best able to migrate from wet soil while Haemonchus contortus was least able to migrate from wet soil.6. It was found that the presence of water on the soil surface tended to lessen the ability of larvae to move on to the grass.7. Haemonchus contortus was found to migrate on relatively dry grass at a faster rate than Trichostrongylus spp. and Ostertagia spp.8. In the temperature range examined larvae were found to move up the grass in largest numbers at 5 and 45° C.9. Haemonchus contortus was the most active species at higher temperatures.10. The cycle of larval movement during the day, under conditions approximating to those of the field, was investigated. The greatest number of larvae were recovered from the grass in early morning. Reactions in the evening were much less marked.