1. The number of third stage larvae of Trichostrongylus retortaeformis on different portions of herbage was studied. Three plant-types were used: Festuca sp., Clover, and Carex sp.

2. On Festuca spp., 5·5 in. in height, 90% of the total number of larvae occurred on the lower 3 in. of the blades; 50% occurred on the basal 1·5 in. On the same species, 3 in. high, 90% were found on the lower inch. This showed that the distribution on the blades was not limited by the total distance the larvae were able to travel. Nor was it limited by the rate of movement, because the form of the distribution was not altered after 3 days.

3. Larvae were found to be evenly distributed vertically over the stems and leaves of clover.

4. On Carex vulgaris 10 in. high the larvae were limited to the lower 2 in.

5. The distribution of larvae in the soil, ‘mat’ and herbage was recorded monthly throughout the year. During the warmer months of June, July and August the majority of larvae were found on the grass blades, and more larvae occurred in the soil than in the ‘mat’. During September, October, April and May more larvae were found in the ‘mat’ and fewer were found on the blades and in the soil. From November to March nearly all the larvae occurred in the ‘mat’. From December to January no larvae were found on the grass blades or in the soil.

6. Climatic conditions in the herbage were studied.

7. It was shown that the temperature below the top of grass blades may differ considerably from the air temperature. Differences of 10° F. were recorded between the tips and bases of grass blades. In general, there was a gradient of temperature established between the upper and lower parts of the grass.

8. High humidities occurred in the lower parts of the grass, even when atmospheric humidity was low. The relative humidity at the tips was never higher than that at the base of the blades.

9. The rate of evaporation decreased towards the base of the blades.

10. The light intensity decreased towards the soil. Midway between the top of the grass and the soil the light intensity fell rapidly.

11. The height, density and type of herbage modified the effect of the climatic factors.

12. It was concluded that the vertical distribution of larvae could be explained without reference to geotropism and that most of the larvae occurred in that portion of the herbage in which there was least climatic change.

The writer wishes to thank Prof. A. D. Hobson for advice and criticism and Mr W. Lyle Stewart, M.R.C.V.S., for encouragement and the provision of facilities, and Dr G. Lapage for helpful criticism and advice in the preparation of the MS. Thanks are also due to King's College, University of Durham, for a grant from the College Post-Graduate Research Fund.